[TASK] Extend and streamline KeyFrameTest

[oliverklee]

• use a per-test subject similar to [CLEANUP] Use a per-test subject for DocumentTest #963
• add a missing @covers annotation for the parent class
• add tests for the line number

Part of #757

[coveralls]

coverage: 53.778%. remained the same
when pulling f03fa14 on task/avoid-shared-subject-keyframe
into 981464c on main.

[JakeQZ]

We should rename getLineNo() to getLineNumber() at some point...