Report "pytestmark = skip" only once

[blueyed]

When using pytestmark = pytest.mark.skip("reason") on a module you will get a SKIP report for every test in that module (in the short test summary info).

There happens some folding already, which could be extended:

pytest/_pytest/skipping.py

Lines 351 to 360 in 6908d93

	def folded_skips(skipped):
	d = {}
	for event in skipped:
	key = event.longrepr
	assert len(key) == 3, (event, key)
	d.setdefault(key, []).append(event)
	l = []
	for key, events in d.items():
	l.append((len(events),) + key)
	return l

[bilderbuchi]

@nicoddemus @RonnyPfannschmidt as the PR is merged this can be closed I think.