Improve job count selection in dav1d_argon.bash

Automatically select an appropriate job count by default instead of requiring explicit configuration.

$ for i in {1..4}; do ../tests/dav1d_argon.bash -t $i; done
2763 files successfully verified in 0m8s (dav1d 1.4.0-4-gb44ef2b filmgrain=1 cpumask=-1)
2763 files successfully verified in 0m8s (dav1d 1.4.0-4-gb44ef2b filmgrain=1 cpumask=-1)
2763 files successfully verified in 0m9s (dav1d 1.4.0-4-gb44ef2b filmgrain=1 cpumask=-1)
2763 files successfully verified in 0m11s (dav1d 1.4.0-4-gb44ef2b filmgrain=1 cpumask=-1)