dartdoc's unit test setup diverges significantly from actual usage

[jcollins-g]

The analyzer and other core components of dartdoc are not set up the same way for unit tests as they are for real dartdoc, leading to situations where code can work in real-life but not in the unit test environment, discouraging the development of unit tests.

Merge analyzer setup for tests with dartdoc's own analyzer setup, eliminate other differences in setup for unit testing, and add tests for uncovered areas.

[jcollins-g]

This was a problem before #1390 and friends but those changes wound up making the situation worse and highlighting the difficulty of adequate automated testing in the current codebase.

[jcollins-g]

This has actually been addressed piecemeal in a number of changes, most notably #1524.