Restore streaming test output to nargo test

[TomAFrench]

We removed the behaviour where tests would be printed to the console as they were started and completed as this would have broken when tests were made to run in parallel in #4484.

This has the downside of when there's a package with many tests such that running the tests take a long time, nargo appears to hang until all of the tests are run and then they're all printed at once. We should improve how we print out the test results so that we can display the test results as they come in.

[TomAFrench]

One option for this would be to use https://github.com/LukasKalbertodt/libtest-mimic as a test harness but we could at the very least pull some code from it without using it as a proper dependency.