Thanks for finding out (and apologies that I've made it so difficult to find crashing tests).
Further investigation shows that the crashing test cases are:

in test_functions.py:  test_stringresult
in test_python_api.py: test_PyOS_snprintf
in test_slicing.py:   test_char_ptr and test_char_ptr_with_free

All in all, there are quite some char problems in the struni branch, but I'm working on them.