Drop $SYSTEMTESTTOP from bin/tests/system/

The $SYSTEMTESTTOP shell variable if often set to .. in various shell
scripts inside bin/tests/system/, but most of the time it is only
used one line later, while sourcing conf.sh. This hardly improves
code readability.

$SYSTEMTESTTOP is also used for the purpose of referencing
scripts/files living in bin/tests/system/, but given that the
variable is always set to a short, relative path, we can drop it and
replace all of its occurrences with the relative path without adversely
affecting code readability.

bin/tests/system/catz/tests.sh

@@ -12,14 +12,14 @@
 set -e
 
 # shellcheck source=conf.sh
-. "$SYSTEMTESTTOP/conf.sh"
+. ../conf.sh
 
 dig_with_opts() {
 	"$DIG" -p "${PORT}" "$@"
 }
 
 rndccmd() (
-	"$RNDC" -c "$SYSTEMTESTTOP/common/rndc.conf" -p "${CONTROLPORT}" -s "$@"
+	"$RNDC" -c ../common/rndc.conf -p "${CONTROLPORT}" -s "$@"
 )
 
 _wait_for_message() (