tests: Rework testglxgetprocaddress.

It now calls glXGetClientString up front to load the dummy vendor library. The
GLX entrypoint generation path has its own test now, so this test can now work
on builds without assembly support.

It now uses glXQueryServerString to test calling a core GLX function. That's a
better test because it returns a predictable value, so we can make sure that it
actually gets dispatched to the vendor library, instead of just checking that
it doesn't crash.

For testing looking up an OpenGL function, it'll call glXGetProcAddress and
directly call __glDispatchGetProcAddress, and make sure that it gets the same
pointer from both. It doesn't try to call the function, since OpenGL
dispatching has its own tests.