I've found one occurrence of this in the CPython codebase, in test_ast.py. Basically it makes sure that the following expression parses and compiles correctly:

f(1,2,c=3,*d,**e)

I doubt that this is to protect against regressions in this specific syntax. More likely it's just trying to create as many of the different argument passing AST nodes as possible in one call (it's the only test for function calls with arguments). It can probably be slightly refactored:

f(1,2,*c,d=3,**e)