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.

README.md

@@ -61,6 +61,12 @@ bison --defines=MachineIndependent/glslang_tab.cpp.h
       -o MachineIndependent/glslang_tab.cpp
 ```
 
+Glslang is adding the ability to test with
+[Google Test](https://github.com/google/googletest) framework. If you want to
+build and run those tests, please make sure you have a copy of Google Tests
+checked out in the `External/` directory:
+`git clone https://github.com/google/googletest.git`.
+
 Programmatic Interfaces
 -----------------------
 
@@ -118,7 +124,11 @@ Testing
 -------
 
 Test results should always be included with a pull request that modifies
-functionality. There is a simple process for doing this, described here:
+functionality. And since glslang is adding the ability to test with
+[Google Test](https://github.com/google/googletest) framework,
+please write your new tests using Google Test.
+
+The old (deprecated) testing process is:
 
 `Test` is an active test directory that contains test input and a
 subdirectory `baseResults` that contains the expected results of the