Restore the @return types of the static analysis tests

[Amoifr]

Fixes #360.

b5addd4 removed the @return annotations from tests/static-analysis/, so the suite only checked the native iterable/string types and no longer what Psalm infers from each assertion. This restores the 142 precise ones (the @return mixed lines are left out, the native types cover them) and regenerates the non-return/ variants, which now check the @psalm-assert narrowing against them.

Restoring them right away caught two things:

• propertyExists() lost the class-string of its argument: unlike propertyNotExists() and methodExists(), it had no @template T as class-string|object. It now has the same template, and Mixin.php is regenerated.
• allNullOrThrows() declared @return iterable<Closure> for an iterable<Closure|null> input, a test mistake that the missing return check had hidden.

Psalm is clean, and without the propertyExists() change it reports 8 errors. PHPUnit is green, ProjectCodeTest included.

[shadowhand]

Thanks!