Doing complex assertions in tests with `DDAssert*`

In #1137 we introduced a bunch of DDAssert* methods for asserting complex types in tests:

• DDAssertReflect + DDAssertNotReflect -- compare values with reflection
• DDAssertJSONEqual + DDAssertJSONNotEqual -- compare JSON encoded strings for both values
• DDAssertDictionariesEqual + DDAssertDictionariesNotEqual -- compare anonymous dictionaries

These should be used everywhere where Equatable is not enough or is not available.