[backport#15891] test: Require standard txs in regtest by default

Summary:
fa89badf887dcc01e5bdece248b5e7d234fee227 test: Require standard txs in regtest (MarcoFalke)
fa9b4191609c3ef75e69d391eb91e4d5c1e0bcf5 test: Add test that mainnet requires standard txs (MarcoFalke)
fa613ca0a8f99c4771859de9e571878530d3ecb5 chainparams: Remove unused fMineBlocksOnDemand (MarcoFalke)

Pull request description:

I don't see a reason why regtest should allow non-standard txs, as it makes testing mainnet behaviour such as #15846 unnecessarily hard and unintuitive.

Of course, testnet policy remains unchanged to allow propagation of non-standard txs.

ACKs for top commit:

ajtowns:
  ACK fa89badf887dcc01e5bdece248b5e7d234fee227

Tree-SHA512: c4c675affb054868850bd2683aa07f4c741a448cbacb2ea8334191e105f426b0790fe6a468be61e9c5880d24154f7bf1c7075051697172dce92180c1bc3a1c90

This is a backport of Core PR15891, plus changing our own tests to allow for non-standard transactions and prevent them from failing under the new rule.

Test Plan:

ninja check
./test_runner.py --extended

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5764