test: add coverage for -rpcwallet cli option

Summary:
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