Bug 19463: Reduce the chance to have TestBuilder generate twice the same value

Bug 19463: Reduce the chance to have TestBuilder generate twice the same value

When an id is generated by TestBuilder (branchcode for instance) and the
size of the generated string is 1, we have too many chances to get
"Violation of unique constraint in Branch".

This patch increases the number of retries to 5.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Amended to make room for follow-up.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>