treediscovery: rename stop() in tests to fix failures on AIX. It seems ksh, the default shell on AIX, does not permit the creation of a function called stop(). test-treediscovery.t and test-treediscovery-legacy.t both fail on AIX with error 'syntax error at line 25 : `(' unexpected'. Fix by renaming stop() in the scripts to tstop(). For completeness rename start() to tstart() to match. Both tests then pass on AIX. Add check for the use of stop() in a shell script to check-code.