Show deselection count before tests are executed

[twmr]

Fixes #1527

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[coveralls]

Coverage decreased (-0.0008%) to 92.638% when pulling df2f019 on thisch:collection_deselection into 063e2da on pytest-dev:features.

[twmr]

@fontealpina, @nicoddemus and @arunksaha: Can you test this PR please and give me some feedback?

@arunksaha: You suggested the --list-deselected parameter in #1527. Are you interested and have time to contribute to pytest and work on this?

[nicoddemus]

@Thisch I tested it and looks great, thanks a ton!

[arunksaha]

@Thisch Thank you for creating and submitting the patch. I am not familiar with the pytest internals, it might take a while for me.

@nicoddemus Thank you fot testing and merging.