test: added tests for M2M relationships, it works

tests/fixtures/m2m.sql

@@ -0,0 +1,14 @@
+INSERT INTO genres (name)
+VALUES ('fantasy'),
+       ('horror'),
+       ('classic');
+
+INSERT INTO book_genres (book_id, genre_id)
+VALUES (1, 1),
+       (1, 3),
+       (2, 1),
+       (2, 3),
+       (3, 1),
+       (3, 3),
+       (4, 2),
+       (4, 3);