Rename some files

- Rename tests.txt into Variables.parserTests.txt
- Rename Variables_settings.php into tests/mwparsertests/Variables.txt - This will result into
  "php phpunit.php --group ParserTests_Variables" being able to execute only those tests. The
  part behind the "_" is always the file name of the parser tests txt file.

Change-Id: Ic70dab308550e9e086d4a5cbb12fe7e9cdef3eb0