tests/test-cat.t

tests: avoid test-hup hanging on AIX

test-hup hangs on AIX. Under ksh89 on AIX (the default shell),

echo Hello; while [ ! -s not-there ]; do true; done

produces no output while the loop executes. Replacing 'true' with 'sleep 0'
fixes, as does using a less broken shell. ksh93 is fine.

Update check-code.py to look for this, and make same change in test-serve.t.
In fact test-serve works fine, probably because of additional commands between
echo and the loop, but that's a subtlety not easy to test for.

  $ hg init
  $ echo 0 > a
  $ echo 0 > b
  $ hg ci -A -m m
  adding a
  adding b
  $ hg rm a
  $ hg cat a
  0
  $ hg cat --decode a # more tests in test-encode
  0
  $ echo 1 > b
  $ hg ci -m m
  $ echo 2 > b
  $ hg cat -r 0 a
  0
  $ hg cat -r 0 b
  0
  $ hg cat -r 1 a
  a: no such file in rev 7040230c159c
  [1]
  $ hg cat -r 1 b
  1