tests: fix command line overflow on ksh

"<command> `hg manifest`" overflows the puny 1024 byte command line length
limit on AIX default shell ksh. Replace with "hg manifest | xargs <command>".

tests/test-check-code-hg.t

@@ -1,9 +1,9 b''
   $ check_code="$TESTDIR"/../contrib/check-code.py
   $ cd "$TESTDIR"/..
 
-  $ "$check_code" `hg manifest` || echo 'FAILURE IS NOT AN OPTION!!!'
+  $ hg manifest | xargs "$check_code" || echo 'FAILURE IS NOT AN OPTION!!!'
 
-  $ "$check_code" --warnings --nolineno --per-file=0 `hg manifest`
+  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0
   contrib/check-code.py:0:
    > #    (r'^\s+[^_ \n][^_. \n]+_[^_\n]+\s*=', "don't use underbars in identifiers"),
    warning: line over 80 characters
@@ -657,4 +657,4 b''
   tests/test-walkrepo.py:0:
    >         print "Found %d repositories when I should have found 3" % (len(reposet),)
    warning: line over 80 characters
-  [1]
+  [123]