Add apt_generated_test/

[koppor]

Reasons for making this change:

The annotation processing creates a separate sub-directory for test cases.

Links to documentation supporting these rule changes:

Did not find it. A developer tested it in Eclipse and manually added that directory.

[shiftkey]

Found some links on the Eclipse bugtracker:

• https://bugs.eclipse.org/bugs/show_bug.cgi?id=539288

Since upgrading to Eclipse Photon, my Maven submodules get unwanted .gitignore files in their roots with an ignore added for .apt_generated_tests/

I've excluded this already in my global git configuration (.gitignore_global) and don't want my to find .gitignore files in my Maven submodules as all ignores are either handled globally or in the root of my Maven project, not at submodule level.

• https://bugs.eclipse.org/bugs/show_bug.cgi?id=531072

The folder for files generated for test sources can be configured in the project settings "Java Compiler" > "Annotation Processing" with the new setting "Generated test source directory:" (default value ".apt_generated_tests"). If no test code is present in the project, no corresponding folder is created or added to the classpath.

This seems like a reasonable inclusion.

[shiftkey]

@koppor thanks!