tests: add missing b'' prefix in test-check-code.t

It now passes on Python 3.

# skip-blame b prefix

Differential Revision: https://phab.mercurial-scm.org/D4250

contrib/python3-whitelist

@@ -51,6 +51,7 b' test-cat.t'
 test-cbor.py
 test-censor.t
 test-changelog-exec.t
+test-check-code.t
 test-check-commit.t
 test-check-execute.t
 test-check-interfaces.py

tests/test-check-code.t

@@ -22,7 +22,7 b' New errors are not allowed. Warnings are'
   >>> commands = []
   >>> with open('mercurial/debugcommands.py', 'rb') as fh:
   ...     for line in fh:
-  ...         m = re.match("^@command\('([a-z]+)", line)
+  ...         m = re.match(b"^@command\('([a-z]+)", line)
   ...         if m:
   ...             commands.append(m.group(1))
   >>> scommands = list(sorted(commands))