Update main resolution tests
The async main resolution has dropped the custom availability checking,
so I am able to collapse the resolution checking into a single test
(deleting the macos-specific one).
Selection of an asynchronous main vs a synchronous main is dependent on
an `-async-main` flag. Due to the scoring mechanism, and that the
specificity of a given resolution doesn't affect the score, a more
general synchronous main will be selected over a more specific
asynchronous main when the `-async-main` flag is not passed.
If the flag is not passed and the program only contains an asynchronous
main, or the asynchronous main is the only main function that is
available given the constraints, then it will be selected.
Conversely, if the flag is set and no valid asynchronous main functions
exist, the main function will resolve to a synchronous main.
This is the same behavior that is used to resolve an synchronous and
asynchronous static function.
