Replaced testpatchentrypoints_gldispatch with three new unit tests.
testgldispatch_static and testgldispatch_generated test dispatching to multiple
vendor libraries through static and generated dispatch stubs, respectively.
testgldispatch_generated tests entrypoint patching and unpatching. This adds
coverage for patching generated stubs in addition to the static ones.
The entrypoint patching functions in tests/dummy/patchentrypoints.c now take
the name of the function to patch, instead of hard-coding it to only patch
glVertex3fv.