Update test to simply generate a normal mainnet configuration file instead of
using a crazy setup where a regtest=1 config file using an includeconf in the
[regtest] section includes another config file that specifies regtest=0,
retroactively switching the network to mainnet.

This setup was fragile and only worked because the triggered InitError happened
early enough that none of the ignored [regtest] options mattered (only
affecting log output).

This is a backport of core#17556

Note that the extra disable_autoconnect is needed because core#22490 was backported before (D10966).

ninja check-functional