coro helper for SKIP() gtest macro

Summary:
Similar to how CO_FAIL() was added, we could also benefit from a CO_SKIP()

Uses same definition as [GTEST_SKIP_](https://github.com/google/googletest/blob/master/googletest/include/gtest/internal/gtest-internal.h#L1354-L1355) but with co_return instead.

Reviewed By: ispeters, capickett

Differential Revision: D28645478

fbshipit-source-id: e92a23b91f8238247ad9d29b67c1a2954f43a38a

folly/experimental/coro/GtestHelpers.h

@@ -178,4 +178,10 @@
  */
 #define CO_FAIL() CO_GTEST_FATAL_FAILURE_("Failed")
 
+/**
+ * Coroutine version of SKIP() which is defined as GTEST_SKIP()
+ */
+#define CO_SKIP(message) \
+  co_return GTEST_MESSAGE_(message, ::testing::TestPartResult::kSkip)
+
 #endif // FOLLY_HAS_COROUTINES