I've heard this story before but I can't find any reference to it in our A320 manuals - but that doesn't mean it's not an known issue! The relevant information could well be buried in a maintenance manual.
It seems unlikely that the system would lock the slats/flaps whilst airborne just because you went a few hours past the required maintenance interval.
What would happen 1 hour into the flight if you (legally) departed towards your home base 9 days and 23hrs after the last test?
I think it is probably a bit of an urban legend based on a misunderstanding of the following A330 issue:
From the A330 MEL:
21-01 Electrical Clock
After any CMC failure, or electrical power interruption, resulting in loss of the UTC
indication (40 H 00, or amber crosses displayed on ECAM SD), initialize the UTC
indication, and the date using the MCDU.
Note: After initialization of the UTC indication on ECAM SD, if the following ECAM cautions are displayed on ECAM E/WD:
F/CTL FLAP TIP BRK FAULT
F/CTL SLAT TIP BRK FAULT
Flight crew should contact the maintenance staff to perform corresponding slats
and/or flaps WTB engagement test (Refer to the AMM) to clear cautions.
From the A330 FCOM 3:
Note: The “SLAT (FLAP) TIP BRK FAULT” warning is triggered when the automatic test has not been performed during the last 10 days.
This warning being classified as a NO GO item in the MMEL it will have to be
corrected prior to the next flight. This can be done on ground by manually
launching the WTB engagement test accessing the CMS through the MCDU
Oddly, this is mentioned in our A330 manuals only and there is nothing at all about it in our A320 books.