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/ixfr/tests.sh

@@ -301,9 +301,9 @@ sub=$!
 $DIG -p ${PORT} ixfr=0 large @10.53.0.3 > dig.out.test$n
 kill $sub
 )
-lines=`grep hostmaster.large dig.out.test$n | wc -l`
+lines=$(grep hostmaster.large dig.out.test$n | wc -l)
 test ${lines:-0} -eq 2 || ret=1
-messages=`sed -n 's/^;;.*messages \([0-9]*\),.*/\1/p' dig.out.test$n`
+messages=$(sed -n 's/^;;.*messages \([0-9]*\),.*/\1/p' dig.out.test$n)
 test ${messages:-0} -gt 1 || ret=1
 if [ $ret != 0 ]; then echo_i "failed"; fi
 status=$((status+ret))