Bug 18999: (QA followup) Minor fixes for tests

Bug 18999: (QA followup) Minor fixes for tests

Removed unneccessary declaration of $budget and changed
Koha::Acquisition::Order->new->insert into
Koha::Acquisition::Order->new->store as requested in tester feedback

Test plan:
1. Go into your koha-shell

2. set the PERL5LIB variable

3. Run t/db_dependent/Budgets.t

All tests should pass

Sponsored-by: Catalyst IT

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

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