Add --skip_cpu and --skip_gpu options to tests

tests/Test.h

Index: tests/Test.h
diff --git a/tests/Test.h b/tests/Test.h
index 2dc7c66f7c2e2833eb2639b26989a73f06be93e0..f2b92400e2c33dcac37dc420ef8bed7dfdeac084 100644
--- a/tests/Test.h
+++ b/tests/Test.h
@@ -64,7 +64,9 @@ namespace skiatest {
 
         static SkString GetResourcePath();
 
-        virtual bool isThreadsafe() const { return true; }
+        virtual bool isThreadsafe() const { return !this->isGPUTest(); }
+
+        virtual bool isGPUTest() const { return false; }
 
     protected:
         virtual void onGetName(SkString*) = 0;
@@ -82,7 +84,7 @@ namespace skiatest {
         GpuTest() : Test() {}
         static GrContextFactory* GetGrContextFactory();
         static void DestroyContexts();
-        virtual bool isThreadsafe() const { return false; }
+        virtual bool isGPUTest() const { return true; }
     private:
     };