Add @coversNothing in places where @covers does not apply

Add @coversNothing in places where @covers does not apply

These tests apply to things that are not relevant to PHP code coverage,
such as testing presence of messages, JSON files, or the PHPUnit tests
themselves.

Using @coversNothing indicates that there is no code here to be covered,
and prevents warnings when using --strict-coverage mode (T152923).

Change-Id: Id89ee2c15a3ce3f10e34b13fb677cd1af75af9e6