We would like to be able to move the kvserver_test tests into subpackages to
start trimming down the kvserver package as a whole. This projected to allow
reducing CI cycle time and, over time, will allow specialized test suites to
live in subpackages (for example, integration tests related to data placement).

The main obstacle is that kvserver_test tests reach into internals of
kvserver using the helpers_test files which add additional accessors, etc.

The proposal is to move any such accessors behind testing interfaces on their
receivers, i.e. (*Replica).Testing().DoSomething(), and then to move all
kvserver_test tests into a subpackage kvinttest (name TBD).

Internal discussion

Jira issue: CRDB-14029