Fix (some of the) warnings in the test suite

This PR fixes three classes of compiler and deprecation warnings in the test suite:

* `expectResult` is currently deprecated and has been replaced with `assertResult`;
* assertions of the form `should be ===` are deprecated in favor of the form `shouldEqual`; and
* `scala.language.postfixOps` was not in scope within the test classes in which postfix operations were actually used.

The fixes for these issues were almost entirely mechanical, but they eliminate many lines of warning output.