* tests/test-fprintf-gnu.h (test_function): Use RETTYPE instead of
'int'.
* tests/test-fprintf-gnu.c (RETTYPE): New macro.
* tests/test-vdprintf-gnu.c (RETTYPE): New macro.
* tests/test-vfprintf-gnu.c (RETTYPE): New macro.
* tests/test-vdzprintf-gnu.c: New file, based on
tests/test-vdzprintf-posix.c.
* tests/test-vdzprintf-gnu.sh: New file, based on
tests/test-vdzprintf-posix.sh.
* modules/vdzprintf-gnu-tests: New file.