tests/glx: Add unit tests for GLX_ARB_create_context GLX protocol

This adds a new tests directory at the top-level and some extra build
infrastructure.  The tests use the Google C++ Testing Framework, and
they will only be built if configure can detect its availability.  The
tests are automatically wired-in to run with 'make check'.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Chad Versace <chad.versace@linux.intel.com>
diff --git a/Makefile b/Makefile
index cf6555c..c738947 100644
--- a/Makefile
+++ b/Makefile
@@ -23,7 +23,7 @@
 
 check:
 	cd src/glsl/tests/ && ./optimization-test
-	@echo "All tests passed."
+	make -C tests check
 
 clean:
 	-@touch $(TOP)/configs/current