Fine-tune code coverage measurement for optional builds

[lukpueh]

Description of issue or feature request:
@joshuagl has recently overhauled sslib's extra dependency handling. This included the configuration of different tests for different sets of installed dependencies (see #200 and #206.)

In the course of a code review we discussed the possibility of different coverage profiles that accurately report the tested lines depending on the relevant code branches. See #200 (comment) ff. for some ideas.

Current behavior:
Code branches that are executed due to missing optional dependency are generally excluded from code coverage.

Expected behavior:
Accurately report the tested lines depending on the relevant code branches for a given build.

[lukpueh]

Also consider code branches being skipped on certain platform, e.g. PySPX on Windows (#518)

[lukpueh]

Also consider code branches being skipped on certain platform, e.g. PySPX on Windows (#518)

... and HSM on Windows (#542)