totalorder* tests: Verify also the function signatures.
* tests/test-totalorder.h: New file, based on tests/test-totalorder.c.
* tests/test-totalorder.c: Check the signature. Include
test-totalorder.h for the rest.
* tests/test-totalorderf.c: Include <config.h>, <math.h>. Check the
signature. Include test-totalorder.h instead of test-totalorder.c.
* tests/test-totalorderl.c: Likewise.
* modules/totalorder-tests (Files): Add test-totalorder.h, signature.h.
* modules/totalorderf-tests (Files): Add test-totalorder.h, signature.h.
Remove test-totalorder.c.
* modules/totalorderl-tests (Files): Likewise.