CLC-3908: adding account summary sanity tests for My Finances Selenium tests

As agreed with the CFV team, I'm adding some basic data sanity checks to CARS account summary data at the UI level.  These changes simply make sure billing summary figures agree with one another on a given account, and I've added the new tests to the existing UID-driven My Finances test.  Once CLC-3794 is resolved on the API side, we will need to enhance the sanity tests to include a comparison of open transactions versus account balance for each UID.  For now I'm leaving that test out since we'll just end up with distracting test failures caused by a known issue.