Add tests

tests/test_operations.py

@@ -0,0 +1,17 @@
+from tinydb import where
+from tinydb.operations import delete, increment, decrement
+
+
+def test_delete(db):
+    db.update(delete('int'), where('char') == 'a')
+    assert 'int' not in db.get(where('char') == 'a')
+
+
+def test_increment(db):
+    db.update(increment('int'), where('char') == 'a')
+    assert db.get(where('char') == 'a')['int'] == 2
+
+
+def test_decrement(db):
+    db.update(decrement('int'), where('char') == 'a')
+    assert db.get(where('char') == 'a')['int'] == 0