Refs #318: Show inheritance for test classes

This turned out to be much harder than it ought to have been
due to Sphinx. We provider shorter import paths for many of
these classes by importing them to their package's __init__.py
and them importing them from there when we use them. This
confused Sphinx greatly; it could not tell that the two possible
names of a class were the same.

Also, if you add :show-inheritance: to an :autoclass: section,
Sphinx insists that it must find a reference to documentation
for every class in the inheritance chain, or else (since we
build with nitpick mode on), the build fails.

In the end, I wrote in most of the classes' doc strings, things
like "Inherits from <link to class>, <link to class>".
Merge remote-tracking branch 'origin/318-show-test-classes-inheritance' into develop