unittest::log() used to be a separable stream of data in unit tests.
It happened to go to the kDefault component, but conceivably we could edit unittest::log() to redirect it elsewhere. The LOGV2 conversion gets rid of unittest::log() lines entirely, in such a way that they literally become kDefault log statements.

We might want to produce a component just for unit test log statements.
Log lines coming from the test itself are of a different nature than log lines coming from the code under test and the distinction is useful. Previously the filename would give some hint as to the statement's origin, but we could codify this with a component id to make filtering easier.