add StartedTestRun type state

[mimir-d]

• this new type disallows users to misuse the run (eg. by emitting run artifacts when the run hasn't been started yet)
• this also shows that there is little value in having a TestRun object around, while not having started, due to the only action possible on it is to start it. It may be beneficial in the future to only allow started objects to exist, instead of having inert objects passed around. This is a slight change in usage, but not very consequential.
• also fix macros tests since this new StartedTestRun pattern highlighted that they were emitting invalid messages

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[mimir-d]

this PR only covers the TestRun, but the pattern should be replicated for all contextual objects (eg. TestStep, MeasurementSeries).

on commenting out the StartedTestRun.scope method

the binding of the returned future lifetime captures to self: &TestRun is, in retrospect, too constraining. That closure should really hold for any for<'a>, but current rust syntax is unable to capture lifetimes in RPIT impl Future in closures. The current solution for this is BoxFuture, but that implies a stack allocation (which may not be a severe consequence in this usecase, but it does make the usage more complicated because the async blocks need to be boxed).

The other option I've considered was to remove the scope method completely and use a Drop impl to output the end message. That however has 2 issues: it doesnt support any customization of the TestRunOutcome and also, the Drop would contain fallible func calls (because I/O). This is a no-go because of these reasons.

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Ultimately I think the boxed future is the way to go here, in order to guarantee that the end messages are emitted (similar to Thread scope() in a way). Any typestate pattern for this would not guarantee that "end" is called.