testing: add -test.example flag to control execution of examples

Also, don't run examples if -test.run is set.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5697069

src/pkg/testing/testing.go

@@ -280,7 +280,7 @@ func Main(matchString func(pat, str string) (bool, error), tests []InternalTest,
 	before()
 	startAlarm()
 	testOk := RunTests(matchString, tests)
-	exampleOk := RunExamples(examples)
+	exampleOk := RunExamples(matchString, examples)
 	if !testOk || !exampleOk {
 		fmt.Println("FAIL")
 		os.Exit(1)