Halo: Add PsrLoggerTestTrait.

This trait contains a test method that checks whether a logger was
correctly stored as property after passing it to the constructor.
Gravity: Adapt for PsrLoggerTestTrait.
L10n: Adapt for PsrLoggerTestTrait.
Shadow: Adapt for PsrLoggerTestTrait.
Spark: Adapt for PsrLoggerTestTrait.
Vortex: Adapt for PsrLoggerTestTrait.
buildsystem: Exclude complete Halo namespace from code coverage.