Tests: Add a simple LibGL render-test

At the moment we just check if we *can* render a simple triangle, we do
not yet actually test if the image is indeed the triangle we wanted.

This test also outputs the rendered image when GL_DEBUG is enabled to a
file called "picture.bmp" for manual verification.

Co-authored-by: sunverwerth <s.unverwerth@serenityos.org>

Tests/LibGL/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(TEST_SOURCES
+    TestRender.cpp
+)
+
+foreach(source IN LISTS TEST_SOURCES)
+    serenity_test("${source}" LibGL LIBS LibGL)
+endforeach()