You can not do it out of the box. You would need an add-on like Power Scripts or Adaptivist Scriptrunner. If you had Adaptivist Scriptrunner you could use the following JQL function
issueFunction in subtasksOf(" fixVersion = \"19 Sep 2017\"") and issuetype = "Testing"
If you had Power Scripts you could write your own JQL function. It is not also very difficult.
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