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/auth/tests.sh

@@ -183,7 +183,7 @@ n=$((n + 1))
 echo_i "check that CHAOS addresses are compared correctly ($n)"
 ret=0
 $DIG $DIGOPTS @10.53.0.1 +noall +answer ch test.example.chaos > dig.out.test$n
-lines=`wc -l < dig.out.test$n`
+lines=$(wc -l < dig.out.test$n)
 [ ${lines:-0} -eq 2 ] || ret=1
 [ $ret -eq 0 ] || echo_i "failed"
 status=$((status + ret))