test: add block 2016 to mock mainnet

The next commit requires an additional mainnet block which changes the difficulty.

Also fix a few minor mistakes in the test (suite):
- rename the create_coinbase retarger_period argument to halving_period. Before bitcoin#31583 this was hardcoded for regtest where these values are the same.
- drop unused fees argument from mine helper

Finally the CPU miner instructions for generating the alternative mainnet chain are expanded.

test/functional/data/mainnet_alt.json

@@ -2014,7 +2014,8 @@
       1231247971,
       1231248071,
       1231248198,
-      1231248322
+      1231248322,
+      1231248621
     ],
     "nonces": [
       2345621585,
@@ -4031,6 +4032,7 @@
       3658502865,
       2519048297,
       1915965760,
-      1183846025
+      1183846025,
+      2713372123
     ]
 }