test: add coverage for -rpcwallet cli option
and add -getinfo coverage with multiple wallets loaded.
The bitcoin-cli -rpcwallet= option is an essential RPC/CLI option when more than one wallet is loaded (see bitcoin-cli -help | grep -A5 rpcwallet or src/bitcoin-cli.cpp::L61) and it currently has no test coverage.
It is not only used by users, but also by the test framework and ~10 test files via get_wallet_rpc().
This PR adds coverage, while simultaneously improving the -getinfo coverage when multiple wallets are loaded.
This is a backport of Core PR18724
Test Plan: ./test/functional/test_runner.py interface_bit*
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D8975