rdf_parser test: add a test with a different set of triples

[atsampson]

See https://github.com/dajobe/raptor/pull/78.

The rdf_parser test was passing on some platforms, presumably because the turtle parser was just reading the same data again and then terminating - which would still cause it to pass the test as it read the same number of triples. Add an extra set of tests with just a single triple, which should catch this.

Also fix the heading for one of the tests, which was in the wrong place so it was only getting printed on failure.