Improve Go Test Runner

[flowirtz]

This PR improves the golang test runner, both in CI but also locally. In particular, it does these three things to achieve that:

👷 Replace go test with gotestsum

Previously our test output was only in JSON - easy to read for a machine, harder to read for a human.
The gotestsum package is a drop-in replacement for go test and very widely used (e.g. by kubernetes).

Three main improvements it brings are:

1. Change the test output format, from compact to verbose with color highlighting.
2. Print a summary of the test run after running all the tests.
3. write a JUnit XML file for integration with CI systems (used for test summary below)

👷 Add test summary for Go tests

GitHub previously introduced job summaries, which among other things, provides richer information like shown below rather than just a simple pass/fail. We are now exporting that information to GHA as well.

⬆️ Bump actions/setup-go@v4 to v5

Previously we kept getting this deprecation warning, which is further explained here. By bumping the action to v5 that's resolved.

[dlicheva]

It looks good! Is it worth adding a note to the README, to tell the user that they need to run make install-go-test-tools before testing?