Make existing unit tests only run on GL contexts

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1869503002

Review URL: https://codereview.chromium.org/1869503002
diff --git a/tests/ReadWriteAlphaTest.cpp b/tests/ReadWriteAlphaTest.cpp
index 64b4ef6..851cf4e 100644
--- a/tests/ReadWriteAlphaTest.cpp
+++ b/tests/ReadWriteAlphaTest.cpp
@@ -33,7 +33,7 @@
     }
 }
 
-DEF_GPUTEST_FOR_RENDERING_CONTEXTS(ReadWriteAlpha, reporter, ctxInfo) {
+DEF_GPUTEST_FOR_GL_RENDERING_CONTEXTS(ReadWriteAlpha, reporter, ctxInfo) {
     unsigned char alphaData[X_SIZE * Y_SIZE];
 
     bool match;