Merge #15485: add rpc_misc.py, mv test getmemoryinfo, add test mallocinfo

Summary:
f13ad1cae0 modify test for memory locked in case locking pages failed at some point (Adam Jonas)
2fa85ebd1c add rpc_misc.py, mv test getmemoryinfo, add test mallocinfo (Adam Jonas)

Pull request description:

Creating the `rpc_misc.py` functional test file to add space for adding tests to a file that doesn't have a lot of coverage.
  - Removing the `getmemoryinfo()` smoke test from wallet basic rather than moving it to keep the wallet decoupled. Feel like testing for reasonable memory allocation values should suffice.
  - Adding coverage for `mallocinfo()`. Introduced standard lib XML parser since the function exports an XML string that describes the current state of the memory-allocation implementation in the caller.

Tree-SHA512: ced30115622916c88d1e729969ee331272ec9f2881eb36dee4bb7331bf633a6810a57fed63a0cfaf86de698edb5162e6a035efd07c89ece1df56b69d61288072

Backport of Core PR15485

Test Plan:

ninja check-functional

Verify rpc_misc.py is run and passes.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5626