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/unknown/tests.sh

@@ -9,7 +9,7 @@
 # See the COPYRIGHT file distributed with this work for additional
 # information regarding copyright ownership.
 
-. $SYSTEMTESTTOP/conf.sh
+. ../conf.sh
 
 status=0
 
@@ -143,7 +143,7 @@ done
 status=`expr $status + $ret`
 
 echo_i "stop and restart secondary"
-$PERL $SYSTEMTESTTOP/stop.pl unknown ns2
+$PERL ../stop.pl unknown ns2
 start_server --noclean --restart --port ${PORT} unknown ns2
 
 # server may be answering queries before zones are loaded,
@@ -167,7 +167,7 @@ $DIFF large.out dig.out > /dev/null || { ret=1 ; echo_i "$DIFF failed"; }
 status=`expr $status + $ret`
 
 echo_i "stop and restart inline secondary"
-$PERL $SYSTEMTESTTOP/stop.pl unknown ns3
+$PERL ../stop.pl unknown ns3
 start_server --noclean --restart --port ${PORT} unknown ns3
 
 # server may be answering queries before zones are loaded,