fate: disable threading for encoding

This explicitly disables threading for encoding as slices are otherwise
automatically activated.  This should be dropped once option resetting
between files is fully implemented.

Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/regression-funcs.sh

@@ -53,7 +53,7 @@ echov(){
 FFMPEG_OPTS="-v 0 -y"
 COMMON_OPTS="-flags +bitexact -idct simple -sws_flags +accurate_rnd+bitexact"
 DEC_OPTS="$COMMON_OPTS -threads $threads"
-ENC_OPTS="$COMMON_OPTS -dct fastint"
+ENC_OPTS="$COMMON_OPTS -threads 1 -dct fastint"
 
 run_ffmpeg()
 {