tests: use raw string in test-check-code.t

To avoid a SyntaxWarning on Python 3.8 due to invalid \ escape.

Differential Revision: https://phab.mercurial-scm.org/D5837

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(b"^@command\('([a-z]+)", line)
+  ...         m = re.match(br"^@command\('([a-z]+)", line)
   ...         if m:
   ...             commands.append(m.group(1))
   >>> scommands = list(sorted(commands))