Bug 1070845 - [sharedtest] unit test refactoring for new version chai.assert api (rebased)

test_agent used very old version chai library version=0.5.3 in the current version=1.9.1 deepEqua
I think it is too brittle to test the actual value of the error message. I intend this to be a human-readable error, not a unique id that is guaranteed not to change. I think it would be better to just test for the presence of an error message instead of its actual value.

And anyway, some of the tests fail for reasons other than invalid output dimensions. So if this test asserts that they all fail with this message, then something must be wrong with the test. Either the test must be failing or it must not be doing what you think it is doing!