gs: Test uniforms and constants in GLSL 1.50 geometry shaders.

These tests are parallel to the equivalently-named tests in
tests/spec/arb_geometry_shader4, except that it verifies that the
uniforms and constants can be used in GLSL 1.50 geometry shaders
rather than ARB_geometry_shader4 geometry shaders.
gs: Test geometry shader that doesn't generate any primitives.

This test is parallel to the equivalently-named test in
tests/spec/arb_geometry_shader4, except that it verifies correct
behaviour in GLSL 1.50 geometry shaders rather than
ARB_geometry_shader4 geometry shaders.