Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.
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.
