Preserve mock side_effect’s in test_pure

Preserve mock side_effect’s in test_pure

The helper method to test if exceptions are propagated or handled would
unset the mock objects side_effect value when its done. This overrides
the original value making the mock not behave as expected as soon as it
is passed into assert_error_propagates.

This changes the helper to store the original side effect and then put
it back once we are done making it throw an exception.

Change-Id: I71900071a1c050913b7283aa743a17c24da1a6b9
Closes-Bug: #1474130