Allow test selection by file:lineno

[blueyed]

pytest tests/test_foo.py:42 should run the (nearest) test at line 42 in tests/test_foo.py.

When it also ignores a trailing colon it would allow for easy copy'n'pasting from failures.

Via #5973:

To get a better overview of how it can work take a look at the Rspec documentation

[nicoddemus]

Great idea!

[aklajnert]

How do we pick the nearest test? Assuming, we have a following file:

def test_1():
     assert True

def test_2():
     assert False

If the user will pick line 3, should it match test_1 or test_2?
What if the line number falls in the fixture definition?

[RonnyPfannschmidt]

picking whitespace or non test functions should result in a collection error and not running any tests

[RonnyPfannschmidt]

s/whitespace/whitespace between test functions/

one must pick a line within the test function body

[blueyed]

In case you are interested: I am taking at stab at this in blueyed#42, since I have "file:lnum" in summaries now (blueyed#38).