net/rpc: fix aliasing in TestAcceptExitAfterListenerClose
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.