Remove tests that fail under CPython 3.5

See http://bugs.python.org/issue19995 for rationale. As micropython currently
aims for Python 3.3 compatibility I have not changed behaviour, but this
change allows the test suite to be run against a newer Python without having
spurious failures.
Merge pull request #663 from Rosuav/floatpercentx

Remove tests that fail under CPython 3.5