Detect runsettings file automatically for assembly invocation and detect coverlet data collector in runsettings

[ViktorHofer]

Fixes #2418
Fixes #2432

Add an auto-detection mechanism that searches for a ".runsettings" in
the test app's directory if no runsettings file was passed in and only
a single test assembly is invoked.

Extend the AlwaysExecute mechanism for ArgumentProcessors so that
processors can run by default without a switch passed in without being a SpecialCommand.

cc @nohwnd

[ViktorHofer]

This needs a slightly different approach as the runsettings is added too late, this should be done before the lazy initialization.

[nohwnd]

This needs a slightly different approach as the runsettings is added too late, this should be done before the lazy initialization.

Does that mean that this is WIP and you will continue on it, or I should change the approach?

2. How do we prevent this from automatically taking the settings if the user does not want them to be applied?

[ViktorHofer]

Does that mean that this is WIP and you will continue on it, or I should change the approach?

Yes this WIP, just converted the PR to a draft one.

How do we prevent this from automatically taking the settings if the user does not want them to be applied?

My assumption is that if a user puts a .runsettings file into an test app's output directory and names that file exactly .runsettings, then this is intentional to make use of the auto-detection. An alternative solution would be to search for a different name, ie default.runsettings.

[ViktorHofer]

cc @MarcoRossignoli for the coverlet discovery change here.