Fix issues running unit tests with newer RBTools and with writable root.

Review Request #12460 — Created July 11, 2022 and submitted — Latest diff uploaded




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

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.