Fix ScalaTest testOnly include option

Fixes com-lihaoyi#3440

The problem is mill define all the test tasks with `explicitlySpecified`
to be `true`. This result this test always be included.
[Code](https://github.com/scalatest/scalatest/blob/main/jvm/core/src/main/scala/org/scalatest/tools/Framework.scala#L267)
in Scalatest for related logic.

testrunner/src/mill/testrunner/TestRunnerUtils.scala

@@ -126,7 +126,7 @@ import scala.jdk.CollectionConverters.IteratorHasAsScala
           yield new TaskDef(
             cls.getName.stripSuffix("$"),
             fingerprint,
-            true,
+            false,
             Array(new SuiteSelector)
           )
       )