Adding tests for admin/bank_transactions_controller

[mmeest]

Added tests for bank_transactions_controller :

• test_new_page_accessible - testing accessibility for new bank transaction method
• test_creates_bank_transaction - testing POST request method 'create' for new bank transaction
• test_bind_invoice_sets_flash_when_invoice_not_found - testing patch request to bind transaction with invalid invoice number
• test_updates_bank_transaction - testing patch request update method to update transactions description, sum