Fix a Gerrit unit test on newer versions of Python 3.7.x.
Review Request #10622 — Created July 9, 2019 and submitted
Python 3.7.x has changed the error for invalid base64 payloads a couple
of times. This means that a developer testing against one version of
Python 3.7.x might see tests pass while a developer testing with another
version might see them fail.
Instead of testing the full, specific string, we're now testing only the
beginning of the string, so we can ignore the specific part of the error
that keeps changing. This is done by moving to
Unit tests pass on Python 3.7.0 and 3.7.3.