Merge #15350: qa: Drop RPC connection if --usecli

Summary:
PR15350 Backport. Fixes --usecli slowdown from D5170 .

6440e61375 qa: Drop RPC connection if --usecli (João Barbosa)

Pull request description:

Drop the RPC connection used in `TestNode.wait_for_rpc_connection` if `--usecli` is set. If the connection is kept and not used the `Connection: close` header is never sent and so the connection only closes due to timeout (30 sec).

It might be sensible to revert e98a9eede2fb48ff33a020acc888cbcd83e24bbf in a follow up, however it changes the shutdown behavior.

Depends on D5170.

Test Plan: ninja check-all ; observe --cli tests aren't particularly slow compared to their normal counterparts

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5171