Simple JUnit test reports for unittest

Context

While waiting for a better solution for #1213 (closed) , this MR adds some XML to scripts/test_wrapper.sh that exposes the unit:* test results in the MR page on gitlab in the same vein as !2776 (merged) and !2742 (merged).

This MR depends on !2836 (merged) as it depends slightly on that one's refactoring of the gitlab config.

To see how it looks with a failing test, have a look at the demonstration MR !2871 (closed).

This MR does increase the granularity of the test results in the MR description somewhat. Before you had only one unit:alltests that is either success/fail. Now, you get success/fail for each call to scripts/test_wrapper.sh, corresponding roughly to each package declaring test targets. However, with a better solution to #1213 (closed), we will have granularity on the test case level.

Manually testing the MR

• Check out this MR page.
• Find a test coming from the unit tests, e.g, unit:non-proto.
• Click it and you should see the URL to read it's logs as well.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• N/A Document any change to the user interface, including configuration parameters (see node configuration)
• N/A Provide automatic testing (see the testing guide).
• N/A For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.