[backport#16465 1/3] test: Generate one block for each send in wallet_import_rescan

Summary:
This ...

• ensures that enough coins are available/spendable, even when more variants are added
• ensures that all mempool txs are mined, even when more variants are added
• makes the test more specific to test that the confirmation height is properly reported and timestamps are correctly handled in the test logic
• prepares the test for a future, where blocks are skipped for rescan if they are deemed irrelevant by a filter (c.f. BIP157)

This is a partial backport of Core PR16465 - part 1 of 3
Commit https://github.com/bitcoin/bitcoin/pull/16465/commits/fac3dcf7d052586548f2100a0d576618a85741f9

Test Plan: ninja && test/functional/test_runner.py wallet_import_rescan.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8054