CI: store merged coverage as an artifact

[lmb]

Running our test suite creates coverage information in coverage.out:

$ ./run-tests.sh 6.1
Fetching linux-6.1-amd64.tgz
Fetching linux-6.1-amd64-selftests-bpf.tgz
...
$ ls -lh coverage.out 
-rw-r--r--. 1 lorenz lorenz 521K Dec  6 14:12 coverage.out

That can be turned into a html report like so:

$ go tool cover -html=coverage.out -o coverage.html
$ ls -lh coverage.html 
-rw-r--r--. 1 lorenz lorenz 899K Dec  6 14:56 coverage.html

We should integrate this into our CI:

• Upload each generated coverage.out as an artifact for that job
• Collect all coverage.out from the various jobs and merge them using go tool covdata merge
• Generate a single .html which shows overall test coverage and upload attach that to the build somehow
• Brownie points if its possible to look at the coverage html by just clicking on a link instead of downloading + extracting

[lmb]

We might also have to investigate whether our merged test coverage in coverage.out is accurate, it's suspiciously low in some files.

[f1ko]

Happy to take it on

[amit-62]

Hello @lmb and @f1ko, can I work on it?

[ti-mo]

Hi @f1ko, still working on this?

[f1ko]

Apologies, didn't get to it yet.

Feel free to work on it!

[amit-62]

Hello @lmb, please review my PR and suggest improvements.
Thank you.

[saiaunghlyanhtet]

@lmb & @ti-mo
Can you review my pr, plz?
#1613