fate: explicitly set the default THREADS value

This makes the default of '1' more explicit than defaulting to '1' in
fate-run.sh and regression-funcs.sh if THREADS is not set.
Fixes the reported thread count in fate-cpu if THREADS is not set.

tests/Makefile

+THREADS = 1
 VREF = tests/vsynth1/00.pgm
 AREF = tests/data/asynth1.sw