patch 8.2.4395: some code lines not covered by tests

Problem:    Some code lines not covered by tests.
Solution:   Add a few more test cases.  Fix getting more than one error for
            invalid assignment.
diff --git a/src/errors.h b/src/errors.h
index d6b82bb..8e2b9da 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -2788,7 +2788,8 @@
 	INIT(= N_("E1077: Missing argument type for %s"));
 // E1078 unused
 // E1079 unused
-// E1080 unused
+EXTERN char e_invalid_assignment[]
+	INIT(= N_("E1080: Invalid assignment"));
 EXTERN char e_cannot_unlet_str[]
 	INIT(= N_("E1081: Cannot unlet %s"));
 #endif