Support alternatives to trio.run

[altendky]

Draft for:

• Generally finish it up and get rid of all the 1/0 etc
• Documentation
• Don't use real QTrio for testing
  • QTrio is excessively onerous when it comes to dependencies and setup for testing
    • PySide2/PyQt5, xvfb, system library dependencies, etc
  • Maybe just create a class with a .run() to set a flag the test can check and otherwise pass on to trio.run()
• Move trio_test() here
• Check docstrings
  • Oh, that's why I didn't write any. There weren't any before...
• Resolution of the multiple-third-party-runs-selected topic at Support alternatives to trio.run #105 (comment).
  • Just use the closest explicit specification

Relates to:

• Support alternatives to trio.run() for pytest-trio trio#1698
• Fixup for pytest-trio support altendky/qtrio#152

[codecov]

Codecov Report

Merging #105 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #105   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          490       578   +88     
  Branches        41        50    +9     
=========================================
+ Hits           490       578   +88     

Impacted Files	Coverage Δ
pytest_trio/_tests/helpers.py	100.00% <100.00%> (ø)
pytest_trio/_tests/test_fixture_mistakes.py	100.00% <100.00%> (ø)
pytest_trio/_tests/test_trio_mode.py	100.00% <100.00%> (ø)
pytest_trio/plugin.py	100.00% <100.00%> (ø)

[altendky]

Thanks for the help @oremanj.

[oremanj]

Feel free to hit the merge button when you're happy with this -- sorry for the delay in reviewing, and thanks for your contribution!