1099771 - Add a unit test to assert correct behavior for reporting invalid checksums.

We had a bug reported against 2.3[0] wherein a module was not being used correctly. It turns out that this was already fixed in 2.4, but there was
still no unit test coverage on the code that caused the exception. This commit adds coverage to that code.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1099771
Merge branch 'rbarlow-1099771' into pulp-2.4