[test] Rename rpc_timewait to rpc_timeout

Summary:

Rename all to the same name with a scripted diff (and some unrelated
cleanups).

Backport of core PR15026:
https://github.com/bitcoin/bitcoin/pull/15026/files

Note that there is an additional change in wallet_dump.py since I
didn't propagated the bug.

Depends on D4559.

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4560