[RFC] consider testpaths config when looking for manage.py

[blueyed]

I've came up with this when using pytest-django with https://github.com/djangonauts/django-rest-framework-gis/ (openwisp/django-rest-framework-gis#123), where it turned out useful to detect and add the tests dir to sys.path - but it's probably not a good idea altogether?! (for the common case)

TODO:

• doc
• tests
• Q: should it only consider testpaths (and not args) if it is configured?

[codecov-io]

Codecov Report

Merging #478 into master will decrease coverage by 6.88%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   92.63%   85.74%   -6.89%     
==========================================
  Files          32       32              
  Lines        1793     1614     -179     
  Branches      153      140      -13     
==========================================
- Hits         1661     1384     -277     
- Misses         94      177      +83     
- Partials       38       53      +15

Flag	Coverage Δ
#dj110	?
#dj111	?
#dj18	?
#dj19	?
#dj20	?
#dj21	?
#django110	81.72% <66.66%> (?)
#django111	83.45% <66.66%> (?)
#django17	79.61% <66.66%> (?)
#django18	80.79% <66.66%> (?)
#django19	79.86% <66.66%> (?)
#mysql_innodb	78.99% <66.66%> (-6.17%)	⬇️
#mysql_myisam	78.93% <66.66%> (-6.07%)	⬇️
#postgres	81.22% <66.66%> (-7.18%)	⬇️
#py27	83.02% <66.66%> (-7.11%)	⬇️
#py34	80.91% <66.66%> (-3.75%)	⬇️
#py35	79.12% <66.66%> (-5.77%)	⬇️
#py36	80.91% <66.66%> (-4.81%)	⬇️
#py37	?
#pypy	81.59% <66.66%> (?)
#pypy3	80.79% <66.66%> (?)
#pytest3	85.74% <66.66%> (?)
#sqlite	81.59% <66.66%> (-5.3%)	⬇️
#sqlite_file	80.29% <66.66%> (-4.37%)	⬇️

Impacted Files	Coverage Δ
pytest_django/plugin.py	67.37% <66.66%> (-18.91%)	⬇️
pytest_django/migrations.py	0% <0%> (-100%)	⬇️
pytest_django/fixtures.py	83.54% <0%> (-13.68%)	⬇️
pytest_django/live_server_helper.py	80.95% <0%> (-11.24%)	⬇️
tests/test_database.py	98.91% <0%> (-0.26%)	⬇️
tests/conftest.py	100% <0%> (ø)	⬆️
tests/test_environment.py	100% <0%> (ø)	⬆️
tests/test_db_setup.py	100% <0%> (ø)	⬆️
tests/test_django_settings_module.py	100% <0%> (ø)	⬆️
... and 5 more

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