Change backticks to the more legible $()

tests/utf8test

@@ -2,7 +2,7 @@
 
 . "${0%/*}/setup" "$@"
 
-if [ "`$VALGRIND $Q $JQ -nf $JQTESTDIR/utf8-truncate.jq`" != "true" ]; then
+if [ "$($VALGRIND $Q $JQ -nf $JQTESTDIR/utf8-truncate.jq)" != "true" ]; then
 	echo "UTF-8 byte sequences that span the jv_load_file read buffer are mangled"
 	exit 1
 fi