Make the Testing task a predecessor of the Bug Fixing task, and use the Finish-to-Finish (FF) predecessor type. When the end of the Testing task moves for any reason, the bug fixing task will track it.
Another nice little feature here is the use of lag- For example if you are bug fixing right up to the last moment of the last day of testing, then you will never be able to test the final fixes. So you might want bug fixing to end one day earlier than the end of Testing (for example), but still track movements in the Testing task: Just add a 1d lag to the above constraint and the lag will work in reverse as its a F-F relationship. Or conversely, to end the subordinate task one day later than the primary task use a negative lag (i.e. -1d), though of course that doesn't make sense for Testing/Bug fixing.