Recent RBTools 3.x releases updated API error messages to be a bit more

useful. This broke a couple unit tests that checked for the resulting

error message.
These tests have been updated to check the current strings, and

dev-requirements.txt has been updated to specifically require the

newer versions.
We had a couple other unit tests that checked for issues writing a

cookie file. They attempted to point to a directory off of / that it

couldn't write to. This assumed the tests were being run in an

environment without a writable /, but that's not the case in CI

environments.
These tests now base their paths off of /dev/null, which should convey

the same sort of error.

Unit tests pass with the newer RBTools.
Attempted to simulate the writable root test in a CI environment. The

real test will happen after this has landed.