llext: tests: fix "mps2/an385/*_mpu" OOM

With the ztest code added, tests were failing with -ENOMEM on the
mps2/an385/*_mpu platform due to alignment requirements.

Increase the llext test heap size to 32Kbytes to avoid this.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>