niti: add covariance checks

Signed-off-by: Jean Privat <jean@pryen.org>
niti: test null receiver on attribute access

Signed-off-by: Jean Privat <jean@pryen.org>
niti: disable interpretor_type_test

some type test are buggy. I do not know why.

Signed-off-by: Jean Privat <jean@pryen.org>
tests: fig base_isa_nullable_vt.nit

Signed-off-by: Jean Privat <jean@pryen.org>