net/http: only report the first leak of each test run
We don't have a way to terminate the leaked goroutines, and we can't
wait forever for them to exit (or else we would risk timing out the
test and losing the log line describing what exactly leaked).
So we have reason to believe that they will remain leaked while we run
the next test, and we don't want the goroutines from the first leak to
generate a spurious error when the second test completes.
This also removes a racy Parallel call I added in CL 476036, which was
flagged by the race detector in the duplicate-suppression check.
(I hadn't considered the potential interaction with the leak checker.)