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.

contrib/check-code.py

@@ -94,6 +94,8 b' utestpats = ['
      "explicit exit code checks unnecessary"),
     (uprefix + r'set -e', "don't use set -e"),
     (uprefixc + r'( *)\t', "don't use tabs to indent"),
+    (uprefixc + r'.*do\s*true;\s*done',
+     "don't use true as loop body, use sleep 0"),
   ],
   # warnings
   []

tests/test-hup.t

@@ -11,7 +11,7 b' Do test while holding fifo open'
   $ (
   > echo lock
   > echo addchangegroup
-  > while [ ! -s .hg/store/journal ]; do true; done
+  > while [ ! -s .hg/store/journal ]; do sleep 0; done
   > kill -HUP $P
   > ) > p
 

tests/test-serve.t

@@ -14,7 +14,7 b''
   >    else
   >        kill `cat hg.pid`
   >    fi
-  >    while kill -0 `cat hg.pid` 2>/dev/null; do true; done
+  >    while kill -0 `cat hg.pid` 2>/dev/null; do sleep 0; done
   > }
 
   $ hg init test