test: Change feature_config_args.py not to rely on strange regtest=0 behavior

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).

test/functional/feature_config_args.py

@@ -7,6 +7,7 @@
 import os
 
 from test_framework.test_framework import BitcoinTestFramework
+from test_framework import util
 
 
 class ConfArgsTest(BitcoinTestFramework):
@@ -41,10 +42,11 @@ class ConfArgsTest(BitcoinTestFramework):
                 conf.write("wallet=foo\n")
             self.nodes[0].assert_start_raises_init_error(expected_msg='Error: Config setting for -wallet only applied on %s network when in [%s] section.' % (self.chain, self.chain))
 
+        main_conf_file_path = os.path.join(self.options.tmpdir, 'node0', 'bitcoin_main.conf')
+        util.write_config(main_conf_file_path, n=0, chain='', extra_config='includeconf={}\n'.format(inc_conf_file_path))
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
-            conf.write('regtest=0\n') # mainnet
             conf.write('acceptnonstdtxn=1\n')
-        self.nodes[0].assert_start_raises_init_error(expected_msg='Error: acceptnonstdtxn is not currently supported for main chain')
+        self.nodes[0].assert_start_raises_init_error(extra_args=["-conf={}".format(main_conf_file_path)], expected_msg='Error: acceptnonstdtxn is not currently supported for main chain')
 
         with open(inc_conf_file_path, 'w', encoding='utf-8') as conf:
             conf.write('nono\n')

test/functional/test_framework/util.py

@@ -334,16 +334,25 @@ def initialize_datadir(dirname, n, chain):
     datadir = get_datadir_path(dirname, n)
     if not os.path.isdir(datadir):
         os.makedirs(datadir)
-    # Translate chain name to config name
+    write_config(os.path.join(datadir, "bitcoin.conf"), n=n, chain=chain)
+    os.makedirs(os.path.join(datadir, 'stderr'), exist_ok=True)
+    os.makedirs(os.path.join(datadir, 'stdout'), exist_ok=True)
+    return datadir
+
+
+def write_config(config_path, *, n, chain, extra_config=""):
+    # Translate chain subdirectory name to config name
     if chain == 'testnet3':
         chain_name_conf_arg = 'testnet'
         chain_name_conf_section = 'test'
     else:
         chain_name_conf_arg = chain
         chain_name_conf_section = chain
-    with open(os.path.join(datadir, "bitcoin.conf"), 'w', encoding='utf8') as f:
-        f.write("{}=1\n".format(chain_name_conf_arg))
-        f.write("[{}]\n".format(chain_name_conf_section))
+    with open(config_path, 'w', encoding='utf8') as f:
+        if chain_name_conf_arg:
+            f.write("{}=1\n".format(chain_name_conf_arg))
+        if chain_name_conf_section:
+            f.write("[{}]\n".format(chain_name_conf_section))
         f.write("port=" + str(p2p_port(n)) + "\n")
         f.write("rpcport=" + str(rpc_port(n)) + "\n")
         f.write("fallbackfee=0.0002\n")
@@ -355,9 +364,7 @@ def initialize_datadir(dirname, n, chain):
         f.write("printtoconsole=0\n")
         f.write("upnp=0\n")
         f.write("shrinkdebugfile=0\n")
-        os.makedirs(os.path.join(datadir, 'stderr'), exist_ok=True)
-        os.makedirs(os.path.join(datadir, 'stdout'), exist_ok=True)
-    return datadir
+        f.write(extra_config)
 
 
 def get_datadir_path(dirname, n):