Move import and watchonly tests to be legacy wallet only in wallet_balance.py
Summary:
Imports and watchonly behavior are legacy wallet only, so make them only run when the test is in legacy wallet mode.
This is a backport of core#18788 [5/16]
https://github.com/bitcoin/bitcoin/pull/18788/commits/a42652ec10c733a5bf37e418e45d4841f54331b4
Test Plan: test/functional/test_runner.py wallet_balance
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10657