CPU stress test missing stressor

A bug reported by Ike on an ARM64 machine highlighted that we're missing the af-alg stressor for the CPU stress test.

Looking back into history, this is the command line w/ stressor list for the old cpu_stress_ng test:
-timeout -s 9 $end_time stress-ng --aggressive --verify --timeout $runtime \
- --metrics-brief --tz --times \
- --af-alg 0 --bsearch 0 --context 0 --cpu 0 \
- --crypt 0 --hsearch 0 --longjmp 0 --lsearch 0 \
- --matrix 0 --qsort 0 --str 0 --stream 0 \
- --tsearch 0 --vecmath 0 --wcs 0

And this is the stressor list from the new unified script for the stress-ng tests:
    stressors = ['bsearch', 'context', 'cpu', 'crypt', 'hsearch', 'longjmp',
                 'lsearch', 'matrix', 'qsort', 'str', 'stream', 'tsearch',
                 'vecmath', 'wcs']

It's just missing af-alg.