Bug 11124: disallow trying to run the staff benchmark script as the DB account

Bug 11124: disallow trying to run the staff benchmark script as the DB account

On step 6 (at least), the circulation and return page redirect to the
select branch page and nothing is done.

The script should die if the user used is the sql administrator account.

Test plan:
Suppose that the sql admin account is root/root and koha/koha a
superlibrarian account.

1/
perl misc/load_testing/benchmark_staff.pl --steps=6
--url=http://admin.koha.local/cgi-bin/koha/
--password="koha" --user="koha"
should produce:
  ...
  Step 6
  ...

2/
perl misc/load_testing/benchmark_staff.pl --steps=1
--url=http://admin.koha.local/cgi-bin/koha/
--password="root" --user="root"
should produce:
Authentication successful
You cannot use the database administrator account to launch this script

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>