Add regression test for enum signedness
Add regression test for enum signedness

Demontrates that the previous commit fixes the regression introducted in 60d796b1cc70b119e87889ce766ef2e45dbf4740.

Now, the following situation:
 - compiler flag '-fshort-enums' is set,
 - an enum with size less than an int is produced,
 - all enum values are postive or null.
does not suffer anymore from sign confusion when accessing an enum value with the generated Python code.

Test matrix:
 - size of enum: 1, 2, 4, 8 bytes.
 - with/without compiler flag '-fshort-enums'

trolldbois/ctypeslib,trolldbois/ctypeslib,trolldbois/ctypeslib