db: add randomized testing framework

Focused randomized tests are useful to supplement the broad randomness of the metamorphic tests. Writing new randomized tests can be toilsome. We should consider writing an internal package to help write new randomized or metamorphic tests.

Jira issue: PEBBLE-133

Start by examining the existing metamorphic tests and current patterns for writing randomized tests in Pebble. Determine the scope of an internal package that would make focused randomized or metamorphic tests less toilsome. Done means the package design and supported testing workflow are agreed and implemented with appropriate coverage.

Written by the indexing model from the issue text.