Added a separate script glxenv.sh for the common environment variables used by
the other GLX tests.
Removed the testx11glvndproto and testglxnscreens tests, since both of them
depend on the x11glvnd X extension, which is no longer used.
Removed the unused GLX_dummy_copy stuff from the GLX_dummy makefile. Also
removed the COMMON_* variables, since they're only ever used once each.
This subtest verifies that the GLX API library's implementation of
glXGetClientString() (which takes the "union" (concatenation) of each
vendor's client string) works as expected.