Gtests can be run on another source tree

The gtest executable accepts a --test-root option to specify
a root directory for test files.  It defaults to the Test directory
in the source tree from which the executable is built.

For example, this lets us run test exectuables built with MinGW on Linux
on a Windows machine with its own copy of the source tree.

gtests/BuiltInResource.FromFile.cpp

@@ -46,7 +46,7 @@ using DefaultResourceTest = GlslangTest<::testing::Test>;
 
 TEST_F(DefaultResourceTest, FromFile)
 {
-    const std::string path = GLSLANG_TEST_DIRECTORY "/baseResults/test.conf";
+    const std::string path = GlobalTestSettings.testRoot + "/baseResults/test.conf";
     std::string expectedConfig;
     tryLoadFile(path, "expected resource limit", &expectedConfig);
     const std::string realConfig = glslang::GetDefaultTBuiltInResourceString();