Fixed mistake in tests from commit 20399083. Refs #19816.

Fixed #19816 -- Pre-evaluate querysets used in direct relation assignments.

Since assignments on M2M or reverse FK descriptors is composed of a `clear()`,
followed by an `add()`, `clear()` could potentially affect the value of the
assigned queryset before the `add()` step; pre-evaluating it solves the problem.

This patch fixes the issue for ForeignRelatedObjectsDescriptor,
ManyRelatedObjectsDescriptor, and ReverseGenericRelatedObjectsDescriptor.
It completes 6cb6e1 which addressed ReverseManyRelatedObjectsDescriptor.

Fixed transaction handling for a number of operations on related objects.

Thanks Anssi and Aymeric for the reviews. Refs #21174.

There may be more than 100 (default maxsize) commonly seen xx-yy values
on some sites. The additional memory consumption isn't significant.

Also added a comment explaining why this cache must have a maxsize.

Refs #22334. Thanks Tim Graham for spotting the issue.

Refs #22334.

Thanks lee at semel.net for the report.

refs #17922.

Thanks to gwahl@fusionbox.com for the report.

Code borrowed from django.contrib.admin.tests. Without this, the class can
throw an exception with something like "The browser appears to have exited
before we could connect. The output was: Error: no display specified"

refs 6ca6c36f

Thanks cjerdonek for the report.

The test throws an error which Python 3.4 doesn't seem to catch as an
expectedFailure.

refs 7476d96f

Thanks coredumperror at gmail.com for the report and Tim Graham
for the review.

Thanks to @loic for the help and @timgraham for the review.

refs #12030.

All signals should now be registered in AppConfig.ready().

Thanks un33k for the report.

Fixed #22335 -- Fixed popen_wrapper error on non-English Windows systems.

refs #12030.