poker: ensure that straight flush > 4OAK
poker: ensure that straight flush > 4OAK

There's a test that 4OAK > full house and that straight flush > full
house but no check comparing straight flushes versus 4OAKs. This means
an incorrect solution could get them flipped and this wouldn't be
detected by the tests.

Fixes #252

robphoenix/exercism-go,exercism/xgo,robphoenix/exercism-go,leenipper/xgo,exercism/xgo,leenipper/xgo