Allow start_clock() to start the very first system task; make MockClock do so

[oremanj]

Preparatory to #1521 in which the order will do anything. As discussed in #265, the autojumper should be the very last thing to get shut down, even after the run_sync_soon task.

This requires some shenanigans because start_clock() runs before the first step of init. We effectively execute the system nursery __aenter__ outside of async context, so as to have a system nursery when start_clock() runs.

[codecov]

Codecov Report

Merging #1579 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1579   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files         111      111           
  Lines       13866    13866           
  Branches     1059     1059           
=======================================
  Hits        13824    13824           
  Misses         27       27           
  Partials       15       15           

Impacted Files	Coverage Δ
trio/_core/_run.py	99.76% <100.00%> (+<0.01%)	⬆️
trio/_core/tests/test_run.py	100.00% <100.00%> (ø)
trio/testing/_mock_clock.py	100.00% <100.00%> (ø)

[njsmith]

What do you think of the idea discussed in the last few paragraphs of this comment, of making MockClock use an instrument instead of a task? #239 (comment)

[oremanj]

Interesting! I had rather glossed over that issue before, but have now gone back and read it in detail.

I think the instrument-based MockClock is going to be challenging to make work in all edge cases without more instrumentation hooks churn than I'm up for right now. (This issue is to unblock #1521, which together with #1543 unblocks #1564 and #1537... the yak stack depth is getting intimidating!) I commented on #239 with some hazards I saw when I sat down to try it.

I certainly don't mind removing the unusual system_nursery initialization in this PR if MockClock later changes to make it unnecessary.

[njsmith]

I'm intrigued by the idea in this PR to make our bootstrap code start by creating a nursery, instead of starting by creating a task. It's not something I've really thought about before.

As far as fixing MockClock goes though, I think #1588 is probably easier to reason about than this PR, and also sets us up better for future features like chaos scheduling?

[oremanj]

Closing this in favor of #1588. If we later need the system nursery to exist earlier for something else, that's what version control is for!