There are a number of tests in SimpleTypes.tdml that have roundTrip="false" so that they only run parse.
They fail if you round trip them because unparsing doesn't work right, or the tests are somehow problematic (numeric precision issues with parsing to/from binary for floats/doubles, also a number of date/time-related bugs.
There are roughly 39 of these.
Tests "decimal_text" and "dateTimeText" are two of these. Search for roundTrip="false" to find them. Note that there are also some tests that won't round trip due to lack of unparser support for binary bigInt and binary decimal. Those are JIRA