Support test coverage

[Sichao25]

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[Sichao25]

We may need to combine pytest-cov and gcov. I will go back on this later.

[Sichao25]

I get the following summary for the C++ code coverage:

lines:     74.1% (13809 out of 18632)
functions: 74.6% (1588 out of 2128)
branches:  38.8% (14631 out of 37663)

My understanding is that gcovr generates a coverage file containing more detailed information, which we can investigate after uploading or printing. However, the upload step is currently guarded for forked repositories for security (followed the example here):

if: (github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository) && matrix.python_api == 'ON'

It seems that there is no way to verify the uploaded coverage details until we actually trigger the upload step. Alternatively, we could remove this upload step and just print the contents of the output file directly in the CI log with something like cat coverage-cpp.txt.

I would assume that uploading the coverage report is the more standard and visible approach, but I’m also fine with printing it in the CI log if there are different opinions.