[backport#15646] [tests] Add test for wallet rebroadcasts

Summary:
The existing wallet_resendwallettransactions.py test only tests the
resendwallettransactions RPC. It does not test whether transactions are
actually rebroadcast, or whether the rebroadcast logic is called on a
timer.

This commit updates the test to not use the resendwallettransactions RPC and
test that transactions are rebroadcast on a timer.

This is a backport of Core PR15646

Test Plan:

cmake .. -GNinja
./test/functional/test_runner.py wallet_resendwallettransactions.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5947