add some isl_multi_aff_lex_* tests
add some isl_multi_aff_lex_* tests

The next commit will change the implementation of these functions.
The tests help to ensure that this change does not break
the functionality.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl