Merge #16681: Tests: Use self.chain instead of 'regtest' in all current tests

1abcecc40c Tests: Use self.chain instead of 'regtest' in almost all current tests (Jorge Timón)

Pull request description:

  Simply avoiding the hardcoded string in more places for consistency.
  It can also allow for more easily reusing tests for other chains other than regtest.

  Separated from #8994 .
  Continues #16509 .

  It is still not complete (ie to be complete, we need the -chain parameter in #16680 and make whether acceptnonstdtxs is allowed for that chain or not customizable for regtest [or for custom chains like in #8994 ] ). But while being incomplete like #16509 , it's quite simple to review and another step forward IMO.

ACKs for top commit:
  Sjors:
    re-ACK 1abcecc. I think it's an improvement even if incomplete and if some PR's might accidentally bring "regtest" back. Subsequent improvements hopefully don't have to touch 16 files.
  elichai:
    Code review ACK 1abcecc40c
  ryanofsky:
    Code review ACK 1abcecc40c.
  ryanofsky:
    Code review ACK 1abcecc40c

Tree-SHA512: 5620de6dab235ca8bd8670d6366c7b9f04f0e3ca9c5e7f87765b38e16ed80c17d7d1630c0d5fd7c5526f070830d94dc74cc2096d8ede87dc7180ed20569509ee

test/functional/feature_logging.py

@@ -16,7 +16,7 @@ class LoggingTest(BitcoinTestFramework):
         self.setup_clean_chain = True
 
     def relative_log_path(self, name):
-        return os.path.join(self.nodes[0].datadir, "regtest", name)
+        return os.path.join(self.nodes[0].datadir, self.chain, name)
 
     def run_test(self):
         # test default log file name