Merge #14958: qa: Remove race between connecting and shutdown on separate connections

Summary:
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

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5174