Add sanity checks to allow script to double as an env test

This could be used as an environment test that verifies that a deployment of stCELO contracts functions correctly after an upgrade. Would be useful if this then also performed additional checks, e.g.

* Is the stCELO-CELO conversion rate as expected, do we get the expected amount on depositing.
* Brainstorm others.

Could also add a flag to disable sanity checks, if not all are necessary for a load test.