recipe/siptester: minimal fix for ImportError

This code error when it's imported, because `install` does not exist on
this class, so the assignment caused NamError.
This is an "emergency commit" to repair the tests on python 3 after
 3a1e5628 (software/slapos-sr-testing: use nxdtest, 2020-10-28)

slapos/recipe/siptester/__init__.py

@@ -35,7 +35,9 @@ class SipTesterRecipe(BaseSlapRecipe):
     self._createRunningWrapper(d)
     return [path, wrapper_path]
 
-  update = install
+  def update(self):
+    return self.install()
+
 
 class ReceiverRecipe(SipTesterRecipe):
   config_template = "pjsua_receiver.conf.in"