Fix GoInspectionTestCase.doTestWithDirectory() to work on Windows.

Because `FileUtil.getRelativePath(File, File)` uses the character specified by `File.pathSeparator` for the returned file path, the returned paths use `'/'` on Linux and `'\\'` on Windows. This causes a number of tests to fail on Windows, because `myFixture.configureFromTempProjectFile(fileName)` requires `'/'` as the path separator.

The issue can be fixed by normalizing the path before passing it to `configureFromTempProjectFile` (as it is done in this commit).