Tests: Bluetooth: Tester: Increase the number of max mesh tx segments

Increasing the number of max tx segments to be able to fit composition
page 0 into composition data status message.

After the subnet bridge config client and server are added to the
tester, the size needed to report composition data page 0 increased.
Current behaviour of the tester is correct, dropping the element when
there is not enough space in data field, however the PTS application
does not handle the composition data status message without element.

Signed-off-by: alperen sener <alperen.sener@nordicsemi.no>