Change folder name that was causing git checkout to fail on Windows.

As discussed in https://github.com/mozilla/dxr/issues/297 I've renamed `tests/test_markup/code/<folder>` to `tests/test_markup/code/&folder&`. I also updated the test file accordingly. I can now checkout the repository on Windows.

The bug report also discussed the possibility of creating the test folders on the fly. Not being familiar with the code base I went for the easy solution :-|