Tests that pcontains works properly

[kozross]

pcontains was subject to a bug (fixed in #719), which required significant reworking to fix. Additionally, most tests of PInterval (and related) functionality currently reside in the old test suite, instead of the test suite in plutarch-ledger-api.

We need to check that pcontains handles a range of cases correctly with PInterval PPosixTime, using property tests, unit tests, or both. We also need to port any old tests to the right suite, and use the new framework.