test_meta_invoice: Add explicit "missing invoice" tests.

I wrote this while debugging something else, and I don't want to let
a good test go to waste.

tests/test_meta_invoice.py

@@ -127,3 +127,15 @@ def test_bad_type_values_on_transaction(hook, acct1, acct2, value):
     )
     actual = {error.message for error in hook.run(txn)}
     assert actual == {expected_msg}
+
+@pytest.mark.parametrize('acct1,acct2', testutil.combine_values(
+    REQUIRED_ACCOUNTS,
+    NON_REQUIRED_ACCOUNTS,
+))
+def test_missing_invoice(hook, acct1, acct2):
+    txn = testutil.Transaction(postings=[
+        (acct2, -25),
+        (acct1, 25),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {"{} missing {}".format(acct1, TEST_KEY)}