make Start() to a no-op if testserver is started
make Start() to a no-op if testserver is started

Resolves [#103](https://github.com/cockroachdb/cockroach-go/issues/103) and [#98](https://github.com/cockroachdb/cockroach-go/issues/98).

This commit is to ensure that when testserver is already started, calling `ts.Start()` is a no-op.

cockroachdb/cockroach-go