Merge coverage from Linux, macOS and Windows for coverage report

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I realised that we have been running coverage tests only on Linux and then uploaded these to coveralls for the coverage report. This is a mostly correct representation but it doesn't show the whole picture with code coverage over the other platforms. Running all tests with code coverage in the regular platform tests and then merging those before upload should improve the correctness of the report. Another benefti is that the static analysis test can stop running tests again just for coverage.

Is it possible to construct a solution with the existing API?

Merging coverage might be the tricky part.

Describe the solution you'd like to see.

Have platform tests run with coverage reporting enabled, merge all profiles and then upload to coveralls.