stop scalactic being added as an unnecessary runtime dependency

guardian/frontend#24833
Scalactic version conflicts were causing compilation failures when compiling tests in frontend

```
java.lang.NoSuchMethodError: 'void org.scalactic.BooleanMacro.<init>(scala.reflect.macros.whitebox.Context, java.lang.String)'
```

build.sbt

@@ -11,7 +11,6 @@ scalacOptions ++= Seq("-feature", "-deprecation")
 
 libraryDependencies ++= Seq(
   commonsIo,
-  scalatic,
   scalaTest,
   http,
   playJson

project/dependencies.scala

@@ -2,8 +2,7 @@ import sbt._
 
 object Dependencies {
   val commonsIo = "org.apache.commons" % "commons-io" % "1.3.2"
-  val scalatic = "org.scalactic" %% "scalactic" % "3.1.1"
-  val scalaTest = "org.scalatest" %% "scalatest" % "3.1.1" % "test"
+  val scalaTest = "org.scalatest" %% "scalatest" % "3.1.1" % Test
   val http = "org.apache.httpcomponents" % "httpclient" % "4.5.12"
 
   val playJson = "com.typesafe.play" %% "play-json" % "2.8.1"