Make parser test discover in core work the same way as it does in
extensions: any file ending with *.txt under tests/parser is run
as a parser test file.
This search is recursive, which is motivation to also move some
unrelated files under tests/parser/preprocess over to
tests/phpunit/data/preprocess where they belong; they are used
by tests/phpunit/includes/parser/PreprocessorTest.php and are
unrelated to the parser test infrastructure.
Change-Id: I8c84b4b853e1309929dceb700aab1e79a598d8ab
d96207ab86