Add tests for handling uint inputs

Unsigned inputs are now allowed for int8 and int16.
These tests make sure they are accepted, as well
as a test that checks they are only accepted if
the integer width matches.

Signed-off-by: Madeleine Dunn <madeleine.dunn@arm.com>
Change-Id: I1c737ecbd215ff4a9e1ff8273ff9904a391bcbbd