* tests/test-vazsprintf-gnu.h: New file, extracted from
tests/test-vazsprintf-gnu.c.
* tests/test-vazsprintf-gnu.c: Include test-vazsprintf-gnu.h.
(test_function): Moved out to tests/test-vazsprintf-gnu.h.
* modules/vazsprintf-gnu-tests (Files): Add tests/test-vazsprintf-gnu.h.
* tests/test-c-vazsprintf-gnu.c: New file.
* tests/test-c-vazsprintf-gnu.sh: New file, based on
tests/test-c-snprintf.sh.
* modules/c-vazsprintf-gnu-tests: New file.