Move tests.settings into the djblets package.
Review Request #12287 — Created May 18, 2022 and discarded
A recent change to django-evolution broke the djblets tests. We were
DJANGO_SETTINGS_MODULE = tests.settings, but this was picking up
tests/settings.pyin django-evolution instead of the one in djblets.
It doesn't appear possible to use a relative import name here. This
change moves the test settings into the djblets module so we can be sure
to load the right one.