Fix incorrect "collected items" report when specifying tests on the command-line

[nicoddemus]

It turns out pytest was triggering the pytest_collectreport hook when collecting the nodes in order to find the item specified in the command-line, but this is incorrect given that those nodes might not end up in the final list of items that will actually be run.

As a bonus this now only reports the exact number of items that will be executed:

class TestClass(object):
    
    def test_main(self):
        assert 1
        
    def test_bla(self):
        assert True

$ pytest .tmp\test_foo.py::TestClass::test_main
============================= test session starts =============================
platform win32 -- Python 3.6.0, pytest-2.0.1, py-1.4.33, pluggy-0.4.0
rootdir: C:\pytest, inifile: tox.ini
plugins: hypothesis-3.7.0
collected 1 item

.tmp\test_foo.py .

========================== 1 passed in 0.01 seconds ===========================

cc @rev112

Fix #2464

[The-Compiler]

I'm wondering if this could break some expectations plugins had in some way? If so, maybe it should go to features?

[nicoddemus]

I wondered about that myself... but I suspect everyone expects that an item seen by pytest_collectreport to also appear later on pytest_collection_modifyitems (for example), which was not True previously.

But I doubt this will break anything because this change only enters in effect if the user is passing a test in the command line explicitly (test_foo.py::test_foo).

[coveralls]

Coverage remained the same at 92.133% when pulling 46d157f on nicoddemus:collection-report-2464 into 5ee9793 on pytest-dev:master.

[nicoddemus]

@The-Compiler @RonnyPfannschmidt gentle ping. 😁

[The-Compiler]

Looks okay to me, but I don't really understand how the hook call change changes things, so I'd rather have @RonnyPfannschmidt take a look too 😉