add a way to test multiple files in one test case in GoInspectionTestCase, add test for #438(not run),add test for import path not found

add a way to test multiple files in one test case in GoInspectionTestCase, 
add test for #438 (not running),
add test for import path not found.

Now we can add InspectionTestCase to test import path or unused variables cross files.


@bronze1man can you please add a TODO FIX ME so that we can scan for it with IDEA and see what we need to fix in the tests? Thanks.