all: use t.Skip{,f}

Replace various t.Log{,f} ; return checks with t.Skip{,f}.

R=golang-dev, n13m3y3r, bradfitz, adg, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7193044
diff --git a/src/pkg/runtime/proc_test.go b/src/pkg/runtime/proc_test.go
index 0bbf9fa..bf97fb1 100644
--- a/src/pkg/runtime/proc_test.go
+++ b/src/pkg/runtime/proc_test.go
@@ -22,8 +22,7 @@
 
 func TestStopTheWorldDeadlock(t *testing.T) {
 	if testing.Short() {
-		t.Logf("skipping during short test")
-		return
+		t.Skip("skipping during short test")
 	}
 	maxprocs := runtime.GOMAXPROCS(3)
 	compl := make(chan bool, 2)