Test result duration is not reset by test restarts or guest reboots

[happz]

Every test start got new Stopwatch instances, new measurement, and only the last one was stored in the corresponding result. The patch does the following:

• Stopwatch.measure() is no longer a classmethod. To be restartable, and retain its original starting point, an instance is needed, and it is really, really hard to make a method both instance and classmethod.
• Stopwatch.measure() does not reset its startign poitn when it's already set. This allows stopwatch to be restartable, continuing the measurement.
• Instead of a fresh one Stopwatch every time a test is started, Stopwatch instance is attached to TestInvocation instance owning the test. execute/tmt then calls this instance's measure()method every time the test is started - thanks to its starting point not beign reset every time,TestInvocation.stopwatch` now records the duration across all reboots and restarts.
• And instead of calling format_duration(), Result construction accessess properties of the aforementioned Stopwatch instance, getting the right strings every time.

Pull Request Checklist

• implement the feature
• include a release note

[gemini-code-assist]

Code Review

This pull request refactors execution timing by replacing class-level stopwatch measurements with instance-level tracking. It introduces a stopwatch instance field on TestInvocation to replace individual start_time, end_time, and real_duration fields, and updates the Stopwatch class to support instance-based measurement with a new started property. I have no feedback to provide as there are no review comments.