testing: do not print 'no tests' when there are examples

I am not sure why RunTests and RunExamples are
exported, but I assume that because they are we
should not change the signature, so I added an
unexported global shared by Main and RunTests.

Fixes #3237.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5779043

src/pkg/testing/testing.go

@@ -107,6 +107,8 @@ var (
 	cpuListStr     = flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
 	parallel       = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
 
+	haveExamples bool // are there examples?
+
 	cpuList []int
 )
 
@@ -279,6 +281,7 @@ func Main(matchString func(pat, str string) (bool, error), tests []InternalTest,
 
 	before()
 	startAlarm()
+	haveExamples = len(examples) > 0
 	testOk := RunTests(matchString, tests)
 	exampleOk := RunExamples(matchString, examples)
 	if !testOk || !exampleOk {
@@ -303,7 +306,7 @@ func (t *T) report() {
 
 func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool) {
 	ok = true
-	if len(tests) == 0 {
+	if len(tests) == 0 && !haveExamples {
 		fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
 		return
 	}