Replace nonexistent function in test

test_extension_manager was trying to use assert_called_once, but this
function is nonexistent, and was being improperly ignored by the mock
interface. This patch fixes that by checking the call count of the
mocked function, and checking it.

Change-Id: Id00b4a7bb58e8ca71ca1d0878f9f2a28f1052f92
Closes-Bug: #1326344
Merge "Replace nonexistent function in test"