Enable test for failure of Vulkan shader targeting OpenGL

Glslang bug 229 has been fixed.
diff --git a/libshaderc_util/src/compiler_test.cc b/libshaderc_util/src/compiler_test.cc
index 2535035..9614f5d 100644
--- a/libshaderc_util/src/compiler_test.cc
+++ b/libshaderc_util/src/compiler_test.cc
@@ -232,9 +232,7 @@
   EXPECT_FALSE(SimpleCompilationSucceeds(kVulkanVertexShader, EShLangVertex));
 }
 
-TEST_F(CompilerTest, DISABLED_VulkanSpecificShaderFailsUnderOpenGLRules) {
-  // This test is disabled due to an apparent bug in Glslang.
-  // https://github.com/KhronosGroup/glslang/issues/229
+TEST_F(CompilerTest, VulkanSpecificShaderFailsUnderOpenGLRules) {
   compiler_.SetMessageRules(kOpenGLRules);
   EXPECT_FALSE(SimpleCompilationSucceeds(kVulkanVertexShader, EShLangVertex));
 }