Add test for HIGH() and LOW()

It compares the results of the operators with the expected result if
doing the same thing manually.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

test/link/test.sh

@@ -1,5 +1,6 @@
 otemp=$(mktemp)
 gbtemp=$(mktemp)
+gbtemp2=$(mktemp)
 outtemp=$(mktemp)
 
 RGBASM=../../rgbasm
@@ -23,4 +24,10 @@ diff romx-tiny-no-t.out $outtemp
 $RGBLINK -t -o $gbtemp $otemp > $outtemp 2>&1
 diff romx-tiny-t.out $outtemp
 
+$RGBASM -o $otemp high-low-a.asm
+$RGBLINK -o $gbtemp $otemp
+$RGBASM -o $otemp high-low-b.asm
+$RGBLINK -o $gbtemp2 $otemp
+diff $gbtemp $gbtemp2
+
 exit 0