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