CI: add code coverage support

[matttbe]

It can be helpful to regularly monitor which part of the code is covered or not by our test suite.

Adding gcov supports should not be difficult to do: https://www.kernel.org/doc/html/latest/dev-tools/gcov.html

Then we can probably use an existing Github Action to publish results:

• Coveralls: https://github.com/marketplace/actions/coveralls-github-action
• ReportGenerator: https://github.com/marketplace/actions/reportgenerator

[matttbe]

Note: maybe we can add GCov support only for files from net/mptcp. A bit similar to what is suggested there for RDS: https://lore.kernel.org/r/20240626012834.5678-1-allison.henderson@oracle.com