* expose `warnings=` to pytester `assert_outcomes()`
* fix test fallout from adding warnings= to assert_outcomes()
* #closes 8593 - Improve test and add a `changelog` entry for the change
An upcoming commit wants to import from `_pytest.pytester` in the public
`pytest` module. This means that `_pytest.pytester` would start to get
imported during import time, which it hasn't up to now -- it was
imported by the plugin loader (if requested). When a plugin is loaded,
it is subjected to assertion rewriting, but only if the module isn't
imported yet, it issues a warning "Module already imported so cannot be
rewritten" and skips the rewriting. So we'd end up with the pytester
plugin not being rewritten, but it wants to be.
Absent better ideas, the solution here is to split the pytester
assertions to their own plugin (which will always only be imported by
the plugin loader) and exclude pytester itself from plugin rewriting.
