releasetools: Rename test names in test_verity_utils.py.

There's a naming convention between the testcase and the class/method
being tested.

Also remove VerifiedBootVersion2HashtreeInfoGenerator as it's merely a
placeholder without telling anything much (e.g. whether it's to be
implemented or working as intended).

Test: python -m unittest test_verity_utils
Change-Id: Ieb156765c865aa551e3882f381e3a6db2cac4cbc