Add test for single structure type envelope ECM case

[jtlangevin]

An overly broad if statement led to incorrect determination of the number of applicable structure types (new vs. existing) for envelope ECMs. The if statement has been amended, but a sample envelope ECM that only applies to a single structure type should be added to the tests in 'ecm_prep_test.py' to ensure that this problem does not re-emerge in the future.