[tests] Small fixups before deprecating generate

Summary:
In advance of deprecating the generate RPC method, make some small
changes to a small number of inidividual test cases:

• make memory checking less prescriptive in wallet_basic.py
• replace calls to generate with generatetoaddress in wallet_keypool.py
• replace calls to generate with generatetoaddress and fixup label issues in wallet_labels.py
• replace calls to generate with generatetoaddress in wallet_multiwallet.py

This is a partial backport of Core PR14468 : https://github.com/bitcoin/bitcoin/pull/14468/commits/c269209336dfa7e4258cfe7d5b0a3bc07b0da3b2

I also added a similar fix for abc-parkedchain.py

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D5969