But Still store the use-case elapsed time as integer.

Followup of b5d386ef.

An use case represents for example the process of creating one Sale
Order including generation of the related Sale Packing List.

It allows to record the number of use cases processed per benchmark
suite and the total time spent on these for each suite.

This allows to Specify a random sleep time (with 'sleep' parameter) to
get more realistic results. Update the example accordingly.

It was involving twisted conversion back and forth between
erp5.util.testbrowser which expected the URL and ERP5 site id whereas
erp5.util.benchmark which only expected the full URL.

Moreover, it was not working if there is no "real" ERP5 site ID, for example
with: http://foobar.org/.