tests: Make the glXMakeCurrent tests work without assembly support.

Change glMakeCurrentTestResults to a GLX function instead of a GL function, so
that it can be dispatched using a vendor-provided stub instead of having to
generate one.

Remove the testglxmclate test, and the --late option in testglxmakecurrent,
since that only makes sense when MakeCurrentTestResults was a GL function.