test: skip math parser tests when missing $wgTexvc

The Math extension has at least one test marked with the parsertest
option 'texvc'.  We now use it to detects whether $wgTexvc points to an
executable binary and would skip such parser tests if not.

'math' option is kept for backward compatibility, just in case.

Math extension has been updated to use this as of
https://gerrit.wikimedia.org/r/#/c/69873/

bug: 49884
Change-Id: I992b5a5b7aac067f1e368cb3eeba224c056b76f9
Merge "test: skip math parser tests when missing $wgTexvc"