Include .test.java files to the test scope

[prolativ]

Is your feature request related to a problem? Please describe.
To make a scala source file a part of the test scope, one can either put it into test directory or use .scala.test file name extension. However, for java sources the second technique (using .test.java) doesn't work, which might be surprising to users.

Describe the solution you'd like
Files with .test.java extension should be considered a part of the test scope sources

Describe alternatives you've considered
Placing .java files in test directory actually works as a workaround

[SethTisue]

Note that anyone who wants at least one of their Java test classes to be public will get "error: class J is public, should be declared in a file named J.java" from javac. But I guess those users can go the test directory route.