You can add two dateTime fields to your form: submittedTime and testerOpenedTime.
Set a rule that sets the submittedTime to now() when the submitter clicks the submit button (but before it actually submits), and create a rule for form open like the following:
Condition: submittedTime is not blank and testerOpenedTime is blank
Rule: set testerOpenedTime to now()
You may need to add other conditional logic to check the roles if someone other than a tester will be opening the form before the tester does.