Fix TEST_DEPENDENCIES documentation

[czambran@…]

The example of the documentation for controlling the creation for test databases seems flat out incorrect. From the example I assumed that by default 'diamonds' will not have any dependencies but after reading the code(​https://github.com/django/django/blob/master/django/test/simple.py#L296), what actually happens is that Django makes all databases dependent on the default one which is how the guarantee that by default the default DB will be created first. If you run the code as specified in the example you should get the Exception ImproperlyConfigured because the diamonds DB will depend on default and the default DB will depend on the diamonds one.

[czambran@…]

Here is my recommendation to make the documentation clearer and the example valid: ​https://github.com/czambran/django/commit/39a79dc0cdec6de6820f10bd6a6919eb001a3ebb

[Tim Graham <timograham@…>]

In f10e9af22759cc287a967bba02c75d2834761f1e:

Fixed #20166 - Corrected TEST_DEPENDENCIES example in testing docs.

Thanks czambran.

[Tim Graham <timograham@…>]

In f7fefb361d044cac897bff86531281b2e2ead83e:

[1.5.x] Fixed #20166 - Corrected TEST_DEPENDENCIES example in testing docs.

Thanks czambran.

Backport of f10e9af227 from master