PR14958 backport, fixup to D5170
4412a59bfe8228698e5b5bbe8bb21c8e8a70d357 qa: Remove race between connecting and shutdown on separate connections (João Barbosa)
Pull request description:
Fixes the error https://github.com/bitcoin/bitcoin/pull/14670#issuecomment-447255352 reported by @ken2812221.
There is a race between RPC stop and another concurrent call in the test framework. The connection must be established and the command `waitfornewblock` running before calling `stop`.
See also https://github.com/bitcoin/bitcoin/pull/14670#issuecomment-447304513.
Depends on D5170 and D5173