testing: wrap long comment line

Requested in CL 31324 review.

Change-Id: Ic81410e07cce07c6f3727bc46d86b6c54c15eca0
Reviewed-on: https://go-review.googlesource.com/31410Reviewed-by: Rob Pike <r@golang.org>

src/testing/testing.go

@@ -520,7 +520,8 @@ func (c *common) Skipf(format string, args ...interface{}) {
 }
 
 // SkipNow marks the test as having been skipped and stops its execution.
-// If a test fails (see Error, Errorf, Fail) and is then skipped, it is still considered to have failed.
+// If a test fails (see Error, Errorf, Fail) and is then skipped,
+// it is still considered to have failed.
 // Execution will continue at the next test or benchmark. See also FailNow.
 // SkipNow must be called from the goroutine running the test, not from
 // other goroutines created during the test. Calling SkipNow does not stop