setup: find a "bin/tahoe" executable to test based on allmydata.__file__ instead...

setup: find a "bin/tahoe" executable to test based on allmydata.__file__ instead of based on the CWD
This means that the tests still work if you are executing them from a CWD other than the src dir -- *if* the "bin/tahoe" is found at os.path.dirname(os.path.dirname(os.path.dirname(allmydata.__file__))).
If no file is found at that location, then just skip the tests of executing the "tahoe" executable, because we don't want to accidentally run those tests against an executable from a different version of tahoe.