Test mail is sent if the user selects 'previous' at mailing step 4

At step 4 (Test) in CiviMail mailing workflow, if 'Previous' button is selected the test mailing is sent.

To recreate:
1. Create a new mailing
2. Proceed to step 4 and ensure a valid email is entered and Send Test box is checked.
3. Select 'previous'

Observe that the test email will be delivered.

One would expect that the email is only sent when the user selects 'Next'. Desired behaviour is not to send the test message.