[tests] Rename rpc_* functional tests

Summary:
Rename only

Partial backport of core PR11774 (commit 81b79f2)

There is a test missing from this backport (rpc_deprecated.py) which has
been removed in D2012.
This test should be added again when a new deprecation occurs and named
accordingly.

Depends on D2379

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2380