tests: unset PYTEST_ADDOPTS with inner pytest runs

[blueyed]

Ref: #412 (comment)

[codecov-io]

Codecov Report

Merging #537 into master will increase coverage by 5.9%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #537     +/-   ##
=======================================
+ Coverage    86.1%    92%   +5.9%     
=======================================
  Files          33     33             
  Lines        1626   1627      +1     
  Branches      140    140             
=======================================
+ Hits         1400   1497     +97     
+ Misses        173     93     -80     
+ Partials       53     37     -16

Flag	Coverage Δ
#dj110	85.12% <100%> (+4.86%)	⬆️
#dj111	88.62% <100%> (+5.29%)	⬆️
#dj18	85.61% <100%> (+5.48%)	⬆️
#dj19	84.81% <100%> (+4.99%)	⬆️
#dj20	84.32% <100%> (+5.29%)	⬆️
#djmaster	84.32% <100%> (+5.29%)	⬆️
#mysql_innodb	84.26% <100%> (+5.36%)	⬆️
#mysql_myisam	84.2% <100%> (+5.36%)	⬆️
#postgres	87.64% <100%> (+6.03%)	⬆️
#py27	89.36% <100%> (+6.09%)	⬆️
#py34	84.81% <100%> (+4.99%)	⬆️
#py35	84.75% <100%> (+4.99%)	⬆️
#py36	84.44% <100%> (+5.29%)	⬆️
#pypy	86.1% <100%> (+5.17%)	⬆️
#pypy3	80.45% <100%> (+4.99%)	⬆️
#sqlite	86.1% <100%> (+5.17%)	⬆️
#sqlite_file	84.08% <100%> (+5.29%)	⬆️

Impacted Files	Coverage Δ
tests/conftest.py	100% <100%> (ø)	⬆️
pytest_django/live_server_helper.py	90.47% <0%> (+9.52%)	⬆️
pytest_django/fixtures.py	97.4% <0%> (+11.68%)	⬆️
pytest_django/plugin.py	86.28% <0%> (+18.9%)	⬆️
pytest_django/migrations.py	100% <0%> (+100%)	⬆️

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