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/Settings.h

@@ -35,6 +35,8 @@
 #ifndef GLSLANG_GTESTS_SETTINGS_H
 #define GLSLANG_GTESTS_SETTINGS_H
 
+#include <string>
+
 namespace glslangtest {
 
 class GlslangInitializer;
@@ -45,6 +47,8 @@ struct GTestSettings {
     // An indicator of whether GTest should write real output to the file for
     // the expected output.
     bool updateMode;
+    // The root directory for test files.
+    std::string testRoot;
 };
 
 extern GTestSettings GlobalTestSettings;