Add check for right Tests traits in ScalaJS and Native (#2874)

## Motivation

People get confused when they do things like:
```scala
import mill._
import mill.scalalib._
import mill.scalajslib._

object root extends ScalaJSModule {
  def scalaVersion: T[String] = "3.3.1"
  def scalaJSVersion: T[String] = "1.14.0"
  object test extends ScalaTests with TestModule.Utest
}
```

which doesn't work since we need to extend `ScalaJSTests` instead of
`ScalaTests`.
Now we crash with an exception:
```
[build.sc] [49/53] compile 
[info] compiling 1 Scala source to /Users/lorenzo/scala/repro/out/mill-build/compile.dest/classes ...
[info] done compiling
[build.sc] [53/53] methodCodeHashSignatures 
mill.api.MillException: root is a `ScalaJSModule`. root.test needs to extend `ScalaJSTests`.
    mill.scalalib.ScalaModule$ScalaTests.$init$(ScalaModule.scala:37)
    millbuild.build$root$test$.<init>(build.sc:8)
    millbuild.build$root$.test$lzycompute$1(build.sc:30)
    millbuild.build$root$.test(build.sc:30)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.base/java.lang.reflect.Method.invoke(Method.java:566)
    mill.resolve.ResolveCore$.$anonfun$resolveDirectChildren0$10(ResolveCore.scala:272)
```

Pull Request: #2874

---------

Co-authored-by: Tobias Roeser <le.petit.fou@web.de>

scalajslib/test/src/mill/scalajslib/ScalaTestsErrorTests.scala

@@ -0,0 +1,31 @@
+package mill.scalajslib
+
+import mill._
+import mill.define.Discover
+import mill.scalalib.TestModule
+import mill.util.TestUtil
+import utest._
+
+object ScalaTestsErrorTests extends TestSuite {
+  object ScalaTestsError extends TestUtil.BaseModule {
+    object scalaTestsError extends ScalaJSModule {
+      def scalaVersion = sys.props.getOrElse("TEST_SCALA_3_3_VERSION", ???)
+      def scalaJSVersion = sys.props.getOrElse("TEST_SCALAJS_VERSION", ???)
+      object test extends ScalaTests with TestModule.Utest
+    }
+
+    override lazy val millDiscover = Discover[this.type]
+  }
+
+  def tests: Tests = Tests {
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        ScalaTestsError.scalaTestsError.test
+      }
+      val message = error.getCause.getMessage
+      assert(
+        message == s"scalaTestsError is a `ScalaJSModule`. scalaTestsError.test needs to extend `ScalaJSTests`."
+      )
+    }
+  }
+}

scalalib/src/mill/scalalib/ScalaModule.scala

@@ -1,7 +1,15 @@
 package mill
 package scalalib
 
-import mill.api.{DummyInputStream, JarManifest, PathRef, Result, SystemStreams, internal}
+import mill.api.{
+  DummyInputStream,
+  JarManifest,
+  MillException,
+  PathRef,
+  Result,
+  SystemStreams,
+  internal
+}
 import mill.main.BuildInfo
 import mill.util.{Jvm, Util}
 import mill.util.Jvm.createJar
@@ -20,6 +28,29 @@ trait ScalaModule extends JavaModule with TestModule.ScalaModuleBase { outer =>
   type ScalaModuleTests = ScalaTests
 
   trait ScalaTests extends JavaModuleTests with ScalaModule {
+    try {
+      if (
+        Class.forName("mill.scalajslib.ScalaJSModule").isInstance(outer) && !Class.forName(
+          "mill.scalajslib.ScalaJSModule$ScalaJSTests"
+        ).isInstance(this)
+      ) throw new MillException(
+        s"$outer is a `ScalaJSModule`. $this needs to extend `ScalaJSTests`."
+      )
+    } catch {
+      case _: ClassNotFoundException => // if we can't find the classes, we certainly are not in a ScalaJSModule
+    }
+    try {
+      if (
+        Class.forName("mill.scalanativelib.ScalaNativeModule").isInstance(outer) && !Class.forName(
+          "mill.scalanativelib.ScalaNativeModule$ScalaNativeTests"
+        ).isInstance(this)
+      ) throw new MillException(
+        s"$outer is a `ScalaNativeModule`. $this needs to extend `ScalaNativeTests`."
+      )
+    } catch {
+      case _: ClassNotFoundException => // if we can't find the classes, we certainly are not in a ScalaNativeModule
+    }
+
     override def scalaOrganization: Target[String] = outer.scalaOrganization()
     override def scalaVersion: Target[String] = outer.scalaVersion()
     override def scalacPluginIvyDeps: Target[Agg[Dep]] = outer.scalacPluginIvyDeps()

scalanativelib/test/src/mill/scalanativelib/ScalaTestsErrorTests.scala

@@ -0,0 +1,31 @@
+package mill.scalanativelib
+
+import mill._
+import mill.define.Discover
+import mill.scalalib.TestModule
+import mill.util.TestUtil
+import utest._
+
+object ScalaTestsErrorTests extends TestSuite {
+  object ScalaTestsError extends TestUtil.BaseModule {
+    object scalaTestsError extends ScalaNativeModule {
+      def scalaVersion = sys.props.getOrElse("TEST_SCALA_3_3_VERSION", ???)
+      def scalaNativeVersion = sys.props.getOrElse("TEST_SCALANATIVE_VERSION", ???)
+      object test extends ScalaTests with TestModule.Utest
+    }
+
+    override lazy val millDiscover = Discover[this.type]
+  }
+
+  def tests: Tests = Tests {
+    test("extends-ScalaTests") {
+      val error = intercept[ExceptionInInitializerError] {
+        ScalaTestsError.scalaTestsError.test
+      }
+      val message = error.getCause.getMessage
+      assert(
+        message == s"scalaTestsError is a `ScalaNativeModule`. scalaTestsError.test needs to extend `ScalaNativeTests`."
+      )
+    }
+  }
+}