tests: guard against obsolete markers in the hg repo itself

If obsolete markers appear in the hg repo (because of enabling evolve),
then tests that run hg against the hg repo itself will see warnings like:

  obsolete feature not enabled but 4 markers found!

As far as I can tell, this only occurs in test-check-code-hg.t -- in
particular, it will -not- show up on tests that run against test-created
repos, as most of the test suite does.

tests/test-check-code-hg.t

@@ -1,6 +1,6 b''
   $ check_code="$TESTDIR"/../contrib/check-code.py
   $ cd "$TESTDIR"/..
-  $ if hg identify -q > /dev/null; then :
+  $ if hg identify -q > /dev/null 2>&1; then :
   > else
   >     echo "skipped: not a Mercurial working dir" >&2
   >     exit 80
@@ -8,7 +8,8 b''
 
 New errors are not allowed. Warnings are strongly discouraged.
 
-  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0 \
+  $ hg manifest 2>/dev/null \
+  >   | xargs "$check_code" --warnings --nolineno --per-file=0 \
   > || false
   tests/test-serve.t:0:
    >   >        kill `cat hg.pid`