Unit test enhancements

This PR provides:
 
* a new function, randname(len=12) that creates a random string of ASCII characters.
* as a convenience, self.bucket_name and self.key_name are set during setup to a new random string.
* a SKIP_POOL option, since the pool stress test can take a long time
* a bucket per test run (self.search_bucket) set to a new name with searching enabled.
* most importantly, it replaces most uses of bucket and key with these new things, so that test results are more resilient to broken or half-working tests, since they're run on a new bucket each time.

This should fix issue #164