Include certificate handling in Locust testing guide

Added code snippet for handling certificates in Locust tests.

Author: Abhinav-Premsekhar

articles/app-testing/load-testing/how-to-test-secured-endpoints.md

@@ -246,8 +246,32 @@ When you run your load test, Azure Load Testing retrieves the client certificate
 For Locust-based tests, you can retrieve the certificate and use it in your tests script. The certificate configured in the load test configuration are available in the `ALT_CERTIFICATES_DIR`. 
 
 ```Python
+endpoint = os.getenv("endpoint") or "localhost"
 cert_dir = os.getenv("ALT_CERTIFICATES_DIR")
-cert_file = open(os.path.join(cert_dir), "cert_name_in_keyvault.pfx")
+
+base_url = "https://" + endpoint
+
+key_path = "client.key.pem"
+crt_path = "client.crt.pem"
+
+
+@events.test_start.add_listener
+def on_test_start(environment, **kwargs):
+    pfx_path = open(os.path.join(cert_dir, "cert_name_in_keyvault.pfx"))
+
+    # Generate the private key
+    subprocess.check_output(
+        f'openssl pkcs12 -in "{pfx_path}" -out "{key_path}" -nocerts -nodes -passin pass:',
+        shell=True,
+        stderr=subprocess.STDOUT,
+    )
+
+    # Generate the public certificate
+    subprocess.check_output(
+        f'openssl pkcs12 -in "{pfx_path}" -out "{crt_path}" -clcerts -nokeys -passin pass:',
+        shell=True,
+        stderr=subprocess.STDOUT,
+    )
 ```
 
 ## Authenticate with a managed identity