Add a test logger channel for test base classes to use

Problem/Motivation

There doesn't appear to be an easy or consistent way for tests to assert that a message has been logged (via a logger channel, not to the screen).

The workaround is to enable the dblog module for any test needing to assert that a message is logged, but this seems overly complicated.

Proposed resolution

Add a test logger channel for kernel and browser tests, and a method assertLog() to the base classes (via a trait). Since log messages need to persist past page requests (for Browser tests anyway), either the state backend, or a cache backend, can be used to store the logs for the duration of the test.

Remaining tasks

User interface changes

API changes

Data model changes