Unify tests for GLSL std450 extended instructions.
Unify tests for GLSL std450 extended instructions.

Previously we had two tests: one checks test to binary, the other
checks round trip. This patch merges the check in the former to
the latter.

pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,antiagainst/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,greg-lunarg/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,LukasBanana/SPIRV-Tools,umar456/SPIRV-Tools,antiagainst/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,greg-lunarg/SPIRV-Tools,umar456/SPIRV-Tools,KhronosGroup/SPIRV-Tools,LukasBanana/SPIRV-Tools,dneto0/SPIRV-Tools,greg-lunarg/SPIRV-Tools,LukasBanana/SPIRV-Tools,umar456/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools