Merge #15891: test: Require standard txs in regtest by default

fa89badf88 test: Require standard txs in regtest (MarcoFalke)
fa9b419160 test: Add test that mainnet requires standard txs (MarcoFalke)
fa613ca0a8 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 fa89badf88

Tree-SHA512: c4c675affb054868850bd2683aa07f4c741a448cbacb2ea8334191e105f426b0790fe6a468be61e9c5880d24154f7bf1c7075051697172dce92180c1bc3a1c90

test/functional/feature_cltv.py

@@ -57,7 +57,11 @@ def cltv_validate(node, tx, height):
 class BIP65Test(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 1
-        self.extra_args = [['-whitelist=127.0.0.1', '-par=1']]  # Use only one script thread to get the exact reject reason for testing
+        self.extra_args = [[
+            '-whitelist=127.0.0.1',
+            '-par=1',  # Use only one script thread to get the exact reject reason for testing
+            '-acceptnonstdtxn=1',  # cltv_invalidate is nonstandard
+        ]]
         self.setup_clean_chain = True
         self.rpc_timeout = 120