test_struct: add test for Struct.pack arity check

Author: cosminbasca

graalpython/com.oracle.graal.python.test/src/tests/test_struct.py

@@ -259,3 +259,13 @@ def test_iter_unpack():
     assert lh(it) == 0
     assert_raises(StopIteration, next, it)
     assert lh(it) == 0
+
+
+def test_pack_varargs():
+    assert struct.Struct(">B").pack(3) == b'\x03'
+    raised = False
+    try:
+        struct.Struct(">B").pack(3, kw=1)
+    except TypeError:
+        raised = True
+    assert raised