Add a new test, testglxcreatecontext to test the various functions for creating
GLX contexts.
It tests the core GLX functions, the GLX_ARB_create_context and
GLX_EXT_no_config_context extensions, and using a vendor-provided extension
function.
The new test is much simpler than trying to use testglxmakecurrent to test both
context creation and MakeCurrent, especially for trying to test multiple
context creation functions.
