Bug 19066: Add tests for Koha::Account->pay

Bug 19066: Add tests for Koha::Account->pay

This patch adds tests for Koha::Account->pay, to make sure the
library_id param is used to set the branchcode attribute. The use of
userenv to set branchcode if library_id not passed is removed.
Responsability is left to the callers to pass library_id.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> SUCCESS: Tests pass!

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>