Update hwui_unit_tests to support running with Vulkan backend.

Added Macros for RENDERTHREAD_TESTS which run these tests using specific backends.
RENDERTHREAD_TESTS - Runs OpenGL, SkiaGL, and SkiaVulkan
RENDERTHREAD_SKIA_TESTS - Runs SkiaGL and SkiaVulkan
RENDERTHREAD_OPENGL_TESTS - Runs OpenGL

Test: manual running of hwui_unit_tests

Change-Id: Ia7420ee7a38803a15e2d58394d14b38cae8208d3
diff --git a/libs/hwui/tests/unit/GradientCacheTests.cpp b/libs/hwui/tests/unit/GradientCacheTests.cpp
index 0ee9647..a3b346f 100644
--- a/libs/hwui/tests/unit/GradientCacheTests.cpp
+++ b/libs/hwui/tests/unit/GradientCacheTests.cpp
@@ -23,7 +23,7 @@
 using namespace android;
 using namespace android::uirenderer;
 
-RENDERTHREAD_TEST(GradientCache, addRemove) {
+RENDERTHREAD_OPENGL_PIPELINE_TEST(GradientCache, addRemove) {
     Extensions extensions;
     GradientCache cache(extensions);
     ASSERT_LT(1000u, cache.getMaxSize()) << "Expect non-trivial size";