Use $(...) notation for subshells in system tests

The changes were mostly done with sed:

find . -name '*.sh' | xargs sed -i 's/`\([^`]*\)`/$(\1)/g'

There have been a few manual changes where the regex wasn't sufficient
(e.g. backslashes inside the `...`) or wrong (`...` referring to docs or
in comments).

bin/tests/system/nsec3/tests.sh

@@ -191,7 +191,7 @@ _check_nsec3_nsec3param() {
 	grep "${ZONE}.*0.*IN.*NSEC3PARAM.*1.*0.*${ITERATIONS}.*${SALT}" "dig.out.test$n.nsec3param.$ZONE" > /dev/null || return 1
 
 	if [ -z "$SALT" ]; then
-		SALT=`awk '$4 == "NSEC3PARAM" { print $8 }' dig.out.test$n.nsec3param.$ZONE`
+		SALT=$(awk '$4 == "NSEC3PARAM" { print $8 }' dig.out.test$n.nsec3param.$ZONE)
 	fi
 	return 0
 }