cmd/go: run examples even if -run is set if -example is also set

Allows one to disable everything but the example being debugged.
This time for sure.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5700079

src/pkg/testing/example.go

@@ -23,7 +23,7 @@ type InternalExample struct {
 }
 
 func RunExamples(matchString func(pat, str string) (bool, error), examples []InternalExample) (ok bool) {
-	if *match != "" {
+	if *match != "" && *matchExamples == "" {
 		return // Don't run examples if testing is restricted: we're debugging.
 	}
 	ok = true