tests: skip fseek/ftell tests if ungetc is broken

tests: skip fseek/ftell tests if ungetc is broken

* m4/ungetc.m4: New file.
* modules/fseek-tests: Split test, so ungetc dependency is
separate from rest of test.
* modules/fseeko-tests: Likewise.
* modules/ftell-tests: Likewise.
* modules/ftello-tests: Likewise.
* tests/test-fseek.c (main): Isolate ungetc dependency.
* tests/test-fseeko.c (main): Likewise.
* tests/test-ftell.c (main): Likewise.
* tests/test-ftello.c (main): Likewise.
* tests/test-fseek2.sh: New file.
* tests/test-fseeko2.sh: Likewise.
* tests/test-ftell2.sh: Likewise.
* tests/test-ftello2.sh: Likewise.