tests: cache the jinja bytecode between unit tests

The jinja templates are compiled once per test session instead of once
per test, using jinja cache system and a pytest fixture.

https://jinja.palletsprojects.com/en/3.1.x/api/#jinja2.FileSystemBytecodeCache

ihatemoney/tests/conftest.py

@@ -1,6 +1,7 @@
 from unittest.mock import MagicMock
 
 from flask import Flask
+from jinja2 import FileSystemBytecodeCache
 import pytest
 
 from ihatemoney.babel_utils import compile_catalogs
@@ -13,11 +14,19 @@ def babel_catalogs():
     compile_catalogs()
 
 
+@pytest.fixture(scope="session")
+def jinja_cache_directory(tmp_path_factory):
+    return tmp_path_factory.mktemp("cache")
+
+
 @pytest.fixture
-def app(request: pytest.FixtureRequest):
+def app(request: pytest.FixtureRequest, jinja_cache_directory):
     """Create the Flask app with database"""
     app = create_app(request.cls)
 
+    # Caches the jinja templates so they are compiled only once per test session
+    app.jinja_env.bytecode_cache = FileSystemBytecodeCache(jinja_cache_directory)
+
     with app.app_context():
         db.create_all()
     request.cls.app = app