Added delete_test_data function

tests/test_api.py

@@ -30,6 +30,17 @@ def add_test_data():
     db.session.commit()
 
 
+def delete_test_data():
+    db.session.query(Exam).delete()
+    db.session.commit()
+
+    db.session.query(Problem).delete()
+    db.session.commit()
+
+    db.session.query(ProblemWidget).delete()
+    db.session.commit()
+
+
 @pytest.fixture()
 def app():
     app = Flask(__name__, static_folder=None)