testing: add a method testing.CoverMode

This makes it possible to avoid tests where coverage affects the test
results by skipping them (or otherwise adjusting them) when coverage
is enabled.

Update #17699

Change-Id: Ifcc36cfcd88ebd677890e82ba80ee3d696ed3d7c
Reviewed-on: https://go-review.googlesource.com/32483Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -283,6 +283,13 @@ func Short() bool {
 	return *short
 }
 
+// CoverMode reports what the test coverage mode is set to. The
+// values are "set", "count", or "atomic". The return value will be
+// empty if test coverage is not enabled.
+func CoverMode() string {
+	return cover.Mode
+}
+
 // Verbose reports whether the -test.v flag is set.
 func Verbose() bool {
 	return *chatty