Abort collection errors from pytest_collection - #15061

Fixes #15028.

The collection-error abort lives in the default pytest_runtestloop. That hook is firstresult, so a plugin that implements it never hits the check and will run tests (or do something else) after collection failed.

Move the testsfailed check into the default pytest_collection. --continue-on-collection-errors is unchanged. --collect-only still aborts, because the raise now happens before pytest_runtestloop returns early.

This is a documented behaviour change for plugins that replace pytest_collection entirely. @bluetech said that's the right place for the check and to treat it as deferred to the next major.