Add check for right Tests traits in ScalaJS and Native

[lolgab]

Motivation

People get confused when they do things like:

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

[lefou]

A great addition to fix some regularly seen user problems. See my remarks below.

[lefou]

Looks good to me.

[lefou]

@lolgab Just to be sure we don't overlook something. Are there definitely no valid use cases to have a ScalaTests module inside a ScalaJSModule? If we are not certain, we at least need some way to not fail the build.

[lolgab]

@lolgab Just to be sure we don't overlook something. Are there definitely no valid use cases to have a ScalaTests module inside a ScalaJSModule? If we are not certain, we at least need some way to not fail the build.

Sorry, I had already merged the pull request.
Anyway, I don't think there are use cases for it.