PluginTestBase is not actually a "base class for plugin API unit tests"

Problem/Motivation

The docblock for PluginTestBase claims that it's the:

Base class for Plugin API unit tests. 

However:

• It's actually a kernel test.
• It has dependencies on User and Node modules.
• Only three things extend it, hardly the whole test coverage for the plugin system.
• It does some interesting mocking of the Block system, because that's what plugins were for back in 2012.

Proposed resolution

Rename it to something that better documents its purpose, and update its docblock accordingly.

Remaining tasks

TBD.

API changes

None.

Data model changes

None.

Release notes snippet

None.