testing: fix SkipNow and FailNow to avoid panic(nil) check

Sorry, too many windows in which to run all.bash.
Fixes build.

TBR=r
CC=golang-codereviews
https://golang.org/cl/55790043

src/pkg/testing/testing.go

@@ -147,6 +147,7 @@ type common struct {
 	output   []byte       // Output generated by test or benchmark.
 	failed   bool         // Test or benchmark has failed.
 	skipped  bool         // Test of benchmark has been skipped.
+	finished bool
 
 	start    time.Time // Time test or benchmark started
 	duration time.Duration
@@ -275,6 +276,7 @@ func (c *common) FailNow() {
 	// it would run on a test failure.  Because we send on c.signal during
 	// a top-of-stack deferred function now, we know that the send
 	// only happens after any other stacked defers have completed.
+	c.finished = true
 	runtime.Goexit()
 }
 
@@ -338,6 +340,7 @@ func (c *common) Skipf(format string, args ...interface{}) {
 // those other goroutines.
 func (c *common) SkipNow() {
 	c.skip()
+	c.finished = true
 	runtime.Goexit()
 }
 
@@ -376,13 +379,12 @@ func tRunner(t *T, test *InternalTest) {
 	// returned normally or because a test failure triggered
 	// a call to runtime.Goexit, record the duration and send
 	// a signal saying that the test is done.
-	var finished bool
 	defer func() {
 		t.duration = time.Now().Sub(t.start)
 		// If the test panicked, print any test output before dying.
 		err := recover()
-		if !finished && err == nil {
-			err = fmt.Errorf("test executed panic(nil)")
+		if !t.finished && err == nil {
+			err = fmt.Errorf("test executed panic(nil) or runtime.Goexit")
 		}
 		if err != nil {
 			t.Fail()
@@ -394,7 +396,7 @@ func tRunner(t *T, test *InternalTest) {
 
 	t.start = time.Now()
 	test.F(t)
-	finished = true
+	t.finished = true
 }
 
 // An internal function but exported because it is cross-package; part of the implementation