add check_migrations fixture

[PetrDlouhy]

@idlesign Now I hope the tests will pass. I have added also test for the case the migrations are missing and treated old Django versions (<2.0) which doesn't have --check option - the fixture will fail with helpful message.

P.S.: It would be quicker if I would not need to wait for the approval to run tests.

[coveralls]

Pull Request Test Coverage Report for Build 3421714413

• 16 of 16 (100.0%) changed or added relevant lines in 2 files are covered.
• 13 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-1.4%) to 96.296%

---

Files with Coverage Reduction	New Missed Lines	%
pytest_djangoapp/fixtures/templates.py	1	90.48%
pytest_djangoapp/compat.py	2	66.67%
pytest_djangoapp/fixtures/request.py	2	89.86%
pytest_djangoapp/configuration.py	8	86.82%
<!--	Total:	13		-->

Totals
Change from base Build 2210969880:	-1.4%
Covered Lines:	286
Relevant Lines:	297

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 3421714413

• 16 of 16 (100.0%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 97.98%

---

Files with Coverage Reduction	New Missed Lines	%
pytest_djangoapp/configuration.py	8	86.82%
<!--	Total:	8		-->

Totals
Change from base Build 2210969880:	0.3%
Covered Lines:	291
Relevant Lines:	297

---

💛 - Coveralls

[idlesign]

Thank you. Merged.

P.S.: It would be quicker if I would not need to wait for the approval to run tests.

Hm, I thought that GitHub does just that after the very first approval for a new contributor.