added mechanism and macros to allow tests to require concrete assertion

For negative cases it makes a difference which assertion is hit, however so far unit tests just used HIVE_REQUIRE_THROW that could raise assertion even as a result of a bug (f.e. setting transaction expiration after signing) which made the test not effective. New mechanism allows checking for concrete assert condition (exact match of stringized expression is required - IMHO acceptable cost of not having to change any assertion in main code). It was experimentally used in tests for collateralized conversion.