tests: set C11, enable pedantic warnings, fix GCC and Clang warnings
- Set the standard of the test suite from
c11. Would be nice to prevent all the GNUisms creeping in the test suite while it's easy to do so.
- Enabled pedantic warnings.
- Fixed all warnings generated by GCC and Clang, except the unused function warnings in
netdb.c. That test looks work-in-progress, didn't wanted to touch it.
- Disabled one test case that used a GNU specific extension (zero-sized arrays).