add test for _Nullable handling

tests/syntax/nullable.i

+/* _Nullable is a macOS-specific qualifier. For now, we just ignore it. */
+
+int * _Nullable test(void) { return (int*)0; }
+
+extern int (* _Nullable _fptr)(void);
+
+int f(int * _Nullable x) { if (x) return *x; else return 0; }

tests/syntax/oracle/nullable.res.oracle

+[kernel] Parsing nullable.i (no preprocessing)
+/* Generated by Frama-C */
+int *test(void)
+{
+  int *__retres;
+  __retres = (int *)0;
+  return __retres;
+}
+
+int f(int *x)
+{
+  int __retres;
+  if (x) {
+    __retres = *x;
+    goto return_label;
+  }
+  else {
+    __retres = 0;
+    goto return_label;
+  }
+  return_label: return __retres;
+}
+
+