TestRPC writes to newServer and newServerAddr guarded with a
sync.Once.
TestAcceptExitAfterListenerClose was overwriting those variables,
which caused the second invocation of TestRPC within a single process
to fail.

A second invocation can occur as a result of running the test with
multiple values for the -cpu flag.

fixes #19001.

Change-Id: I291bacf44aefb49c2264ca0290a28248c026f80e
Reviewed-on: https://go-review.googlesource.com/36624Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>