What's the problem this feature will solve?

I'd like to be able to use pytester.RunResult.assert_outcomes() to check deselected count.

Describe the solution you'd like

Add a deselected parameter to pytester.RunResult.assert_outcomes()

Plugins that use pytest_collection_modifyitems to change the items and add change the deselected items need to be tested. Using assert_outcomes() to check the deselected count would be helpful.

Alternative Solutions

Use parseoutcomes() instead of assert_outcomes(). parseoutcomes() returns a dictionary that includes deselected, if there are any.
However, if we have a series of tests, some that care about deselected, and some that don't, then we may have some tests using assert_outcomes() and some using parseoutcomes(), which is slightly annoying.

Additional context