sql: move query result checking in testutils/sqlutils

Moving code to verify the result of a Query to sqlutils, where it can be used by
other tests. In the descriptor mutation test we now check the number of keys
against known values rather than relying on checkQueryResponse to count the
number of NULLs in the expected result (which we know).

Embedding a SQLRunner in mutationTest which allows us to use the new function
directly, as well as the other Fatal-on-error wrappers.