Drop $SYSTEMTESTTOP from bin/tests/system/

This was suggested by @ondrej in !7 (merged).

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 could ponder dropping it altogether and replacing all of its occurrences with the relative path without really adversely affecting code readability.