tests: add test that @commands in debugcommands.py are sorted

I felt like inline Python in test-check-code.t was the most
appropriate place for this, as other linters in contrib/ seem to
be source file agnostic.

The test currently fails.

tests/test-check-code.t

@@ -1,20 +1,37 b''
 #require test-repo
 
   $ . "$TESTDIR/helpers-testrepo.sh"
   $ check_code="$TESTDIR"/../contrib/check-code.py
   $ cd "$TESTDIR"/..
 
 New errors are not allowed. Warnings are strongly discouraged.
 (The writing "no-che?k-code" is for not skipping this file when checking.)
 
   $ hg locate -X contrib/python-zstandard | sed 's-\\-/-g' |
   >   xargs "$check_code" --warnings --per-file=0 || false
   Skipping hgext/fsmonitor/pywatchman/__init__.py it has no-che?k-code (glob)
   Skipping hgext/fsmonitor/pywatchman/bser.c it has no-che?k-code (glob)
   Skipping hgext/fsmonitor/pywatchman/capabilities.py it has no-che?k-code (glob)
   Skipping hgext/fsmonitor/pywatchman/msc_stdint.h it has no-che?k-code (glob)
   Skipping hgext/fsmonitor/pywatchman/pybser.py it has no-che?k-code (glob)
   Skipping i18n/polib.py it has no-che?k-code (glob)
   Skipping mercurial/httpclient/__init__.py it has no-che?k-code (glob)
   Skipping mercurial/httpclient/_readers.py it has no-che?k-code (glob)
   Skipping mercurial/statprof.py it has no-che?k-code (glob)
+
+@commands in debugcommands.py should be in alphabetical order.
+
+  >>> import re
+  >>> commands = []
+  >>> with open('mercurial/debugcommands.py', 'rb') as fh:
+  ...     for line in fh:
+  ...         m = re.match("^@command\('([a-z]+)", line)
+  ...         if m:
+  ...             commands.append(m.group(1))
+  >>> scommands = list(sorted(commands))
+  >>> for i, command in enumerate(scommands):
+  ...     if command != commands[i]:
+  ...         print('commands in debugcommands.py not sorted; first differing '
+  ...               'command is %s; expected %s' % (commands[i], command))
+  ...         break
+  commands in debugcommands.py not sorted; first differing command is debugbuilddag; expected debugapplystreamclonebundle