Fix python tests runner

Author: alexisbcc

tools/snippet-testing/language_handler/python.rb

@@ -16,7 +16,15 @@ def self.run_before_test(directory)
     end
 
     private
-
+    def text_with_custom_header(file_content)
+      cert_path = ENV['FAKE_CERT_PATH']
+      file_content.prepend(
+        "import twilio.rest.resources.base\n"\
+        "import sys\n"\
+        "twilio.rest.resources.base.get_cert_file = lambda: '#{cert_path}'\n"\
+        "sys.modules['twilio.rest.base.resources'] = twilio.rest.resources.base\n"
+      )
+    end
     def execute_command(file)
       command = bash_string_command(
         'source /usr/local/bin/virtualenvwrapper.sh &&'\