tests: fix for POSIX compliance
* Mac ls will exit with a different message and exit code [1], replace with
grep (which is what mercurial tests do)
* POSIX echo has no -n switch, use printf instead
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null