[Kernel] add test with negative-width bitfield

tests/syntax/oracle/struct_with_function_field_invalid.res.oracle

-[kernel] Parsing tests/syntax/struct_with_function_field_invalid.i (no preprocessing)
-[kernel] tests/syntax/struct_with_function_field_invalid.i:8: User Error: 
-  field `f' declared as a function
-[kernel] User Error: stopping on file "tests/syntax/struct_with_function_field_invalid.i" that has
-  errors.
-[kernel] Frama-C aborted: invalid user input.

tests/syntax/oracle/struct_with_invalid_field.res.oracle

+[kernel] Parsing tests/syntax/struct_with_invalid_field.i (no preprocessing)
+[kernel] tests/syntax/struct_with_invalid_field.i:8: User Error: 
+  field `f' declared as a function
+[kernel] tests/syntax/struct_with_invalid_field.i:14: User Error: 
+  negative bitfield width (-1)
+[kernel] tests/syntax/struct_with_invalid_field.i:15: User Error: 
+  negative bitfield width (-2147483648)
+[kernel] User Error: stopping on file "tests/syntax/struct_with_invalid_field.i" that has errors.
+[kernel] Frama-C aborted: invalid user input.

tests/syntax/struct_with_function_field_invalid.i → tests/syntax/struct_with_invalid_field.i

@@ -7,3 +7,11 @@
 struct {
   void f(int);
 } s;
+
+// negative-width bitfields, parsing should fail
+struct neg_bf
+{
+  int a:(-1); // invalid
+  int b:(-2147483647-1); // invalid
+  int d:(-0); // valid
+};