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_loadblock.py

@@ -38,7 +38,7 @@ class LoadblockTest(BitcoinTestFramework):
         cfg_file = os.path.join(data_dir, "linearize.cfg")
         bootstrap_file = os.path.join(self.options.tmpdir, "bootstrap.dat")
         genesis_block = self.nodes[0].getblockhash(0)
-        blocks_dir = os.path.join(data_dir, "regtest", "blocks")
+        blocks_dir = os.path.join(data_dir, self.chain, "blocks")
         hash_list = tempfile.NamedTemporaryFile(dir=data_dir,
                                                 mode='w',
                                                 delete=False,