tests: drivers: can: timing: exclude 20kbit/s test for ESP32 TWAI

Exclude the 20kbit/s CAN timing test from running on the Espressif ESP32
TWAI driver. The TWAI does not support bitrates below 50kbit/s due to the
combination of CAN clock and prescaler range.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>