ff44cae279 test: Change feature_config_args.py not to rely on strange regtest=0 behavior (Russell Yanofsky)
Pull request description:
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This change was originally made as part of #17493
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Tree-SHA512: 3f77305454f04438493dfc2abd78a00434b30869454d1c3f54587b9c1f63239c49c90fb3b4d3a777ad130f2184e0f2dac87cee4cd23c50f1b3496a375943da01