Fix Attoparsec test coverage check compatibility

The hpc tool, on squeeze, does not handle utf8 encoded files correctly.
The Attoparsec test is just needed to verify whether the parser handles
uft8 correctly, so the solution is to exclude it from coverage checks.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>