Add option to repeatly execute betterproto operations in test, to evaluate performance

conftest.py

@@ -0,0 +1,10 @@
+import pytest
+
+
+def pytest_addoption(parser):
+    parser.addoption("--repeat", type=int, default=1, help="repeat the operation multiple times")
+
+
+@pytest.fixture(scope="session")
+def repeat(request):
+    return request.config.getoption("repeat")