Bug 11457: avoid spurious test failure in t/db_dependent/Bookseller.t

This patch clears the subscription table so tests pass correctly if
there happens to have been a subscription with an end date before
2013-08-01.

To test
1/ Add a subscription in the serials module, with an end date before
2013-08-01
2/ run prove -v t/db_dependent/Bookseller.t
   Notice tests 22 and 23 fail
3/ Apply patch
4/ run test again
5/ Notice tests succeed

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Tests now clean the subscription table before running
the tests, so that the will always pass.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>