Link in Google Test framework.

The existing test harness is a homemade shell script. All the tests
and the expected results are written in plain text files. The harness
just reads in a test, invoke the glslangValidator binary on it, and
compare the result with the golden file. All tests are kinda
integration tests.

This patch add Google Test as an external project, which provides a
new harness for reading shader source files, compile to SPIR-V, and
then compare with the expected output.

gtests/CMakeLists.txt

@@ -0,0 +1,32 @@
+if (TARGET gmock)
+  message(STATUS "Google Mock found - building tests")
+
+  set(TEST_SOURCES
+    # Framework related source files
+    ${CMAKE_CURRENT_SOURCE_DIR}/Initializer.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/main.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/Settings.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/Settings.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/TestFixture.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/TestFixture.h
+
+    # Test related source files
+    ${CMAKE_CURRENT_SOURCE_DIR}/AST.FromFile.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/BuiltInResource.FromFile.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/Pp.FromFile.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/Spv.FromFile.cpp
+  )
+
+  add_executable(glslangtests ${TEST_SOURCES})
+  target_compile_definitions(glslangtests
+    PRIVATE GLSLANG_TEST_DIRECTORY="${CMAKE_CURRENT_SOURCE_DIR}/../Test")
+  target_include_directories(glslangtests PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}
+    ${PROJECT_SOURCE_DIR}
+    ${gmock_SOURCE_DIR}/include
+    ${gtest_SOURCE_DIR}/include)
+  target_link_libraries(glslangtests PRIVATE
+    glslang OSDependent OGLCompiler glslang
+    SPIRV glslang-default-resource-limits gmock)
+  add_test(NAME glslang-gtests COMMAND glslangtests)
+endif()