Make it possible to pass in paths to opus_demo and opus_compare.

This patch lets you pass paths for opus_demo and opus_compare to the
run_vectors.sh script.

The motivation for this was for my aarch64 patches (forthcoming),
which I've been testing under qemu.  I want to run an emulated
opus_demo, but a native opus_compare, so the script completes in a
reasonable amount of time.

tests/run_vectors.sh

@@ -45,8 +45,8 @@ CMD_PATH=$1
 VECTOR_PATH=$2
 RATE=$3
 
-OPUS_DEMO=$CMD_PATH/opus_demo
-OPUS_COMPARE=$CMD_PATH/opus_compare
+: ${OPUS_DEMO:=$CMD_PATH/opus_demo}
+: ${OPUS_COMPARE:=$CMD_PATH/opus_compare}
 
 if [ -d $VECTOR_PATH ]; then
     echo Test vectors found in $VECTOR_PATH