Fix crash in _check_table_difference() when table hashes aren't equal

[gizmoguy]

Fixes this crash in _check_table_difference():

  File "/faucet-src/clib/valve_test_lib.py", line 787, in _check_table_difference
    msg="%s != %s\n".join(diff) % (before_hash, after_hash),
        ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
TypeError: not enough arguments for format string

Also as it turns out, hashing of FakeOFTable objects was broken which meant that _check_table_difference() would not always detect a difference between two different table states. The hash was created based on a frozenset of a string:

return hash(frozenset(str(self.tables)))

This effectively just counted unique characters in the string representation of a table, and only if there was a new character present in one of the table states would a different hash be calculated.

Also made a few formatting improvements, firstly table_state() was modified to return the prettified representation of table state which matches the format that _check_table_difference() uses for diffing. Secondly, the order in which flowmods are printed for a table is now sorted by priority then by match.