Improve docs for running tests using a different database

[Anna Schneider]

For new contributors working on contrib.postgres, it is confusing when the tests in postgres_tests are skipped by default. Some possible improvements include:

In the "skipped tests" paragraph of ​https://docs.djangoproject.com/en/1.9/intro/contributing/#running-django-s-test-suite-for-the-first-time, mention that another reason that tests might get skipped is if the tests are specific to a non-sqlite database. Add a link to ​https://docs.djangoproject.com/en/1.9/internals/contributing/writing-code/unit-tests/#using-another-settings-module to help people use a different settings module.

In ​https://docs.djangoproject.com/en/1.9/internals/contributing/writing-code/unit-tests/#using-another-settings-module, clarify that the default settings module is located at django/tests/test_sqlite.py.

[krisys]

Greetings,

aschn and timgraham, I just updated the documentation to reflect the suggestions and I have created a pull request for the same - ​https://github.com/django/django/pull/6418

Thank you.

[Tim Graham <timograham@…>]

In 361cb7a:

Fixed #26448 -- Added details for running tests with a different database backend.