fix: fixed sales return and credit note failure

[vishdha]

• Fixes Test Case

Traceback/Bug Report:

1. FAIL: test_payment_against_negative_sales_invoice (erpnext.accounts.doctype.payment_entry.test_payment_entry.TestPaymentEntry)
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Traceback (most recent call last):
  File "/home/vishal/digi_frappe_sprint/apps/erpnext/erpnext/accounts/doctype/payment_entry/test_payment_entry.py", line 356, in test_payment_against_negative_sales_invoice
    self.assertEqual(si1_outstanding, -100)
AssertionError: 0.0 != -100

2.======================================================================
FAIL: test_accounts_receivable (erpnext.accounts.report.accounts_receivable.test_accounts_receivable.TestAccountsReceivable)
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Traceback (most recent call last):
  File "/home/vishal/digi_frappe_sprint/apps/erpnext/erpnext/accounts/report/accounts_receivable/test_accounts_receivable.py", line 54, in test_accounts_receivable
    [row.invoice_grand_total, row.invoiced, row.paid, row.credit_note, row.outstanding])
AssertionError: Lists differ: [100, 0, 0, 40, -40] != [100.0, 50.0, 10.0, 0.0, 40.0]

First differing element 1:
0
50.0

- [100, 0, 0, 40, -40]
+ [100.0, 50.0, 10.0, 0.0, 40.0]


======================================================================
FAIL: test_credit_note (erpnext.accounts.doctype.sales_invoice.test_sales_invoice.TestSalesInvoice)
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Traceback (most recent call last):
  File "/home/vishal/digi_frappe_sprint/apps/erpnext/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py", line 1563, in test_credit_note
    self.assertEqual(si.outstanding_amount, outstanding_amount)
AssertionError: -2500.0 != 0.0

4.======================================================================
FAIL: test_return_sales_invoice (erpnext.accounts.doctype.sales_invoice.test_sales_invoice.TestSalesInvoice)
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Traceback (most recent call last):
  File "/home/vishal/digi_frappe_sprint/apps/erpnext/erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py", line 1076, in test_return_sales_invoice
    self.assertEqual(party_credited, 1000)
AssertionError: None != 1000
5. ======================================================================
FAIL: test_default_bank_account (erpnext.erpnext_integrations.doctype.plaid_settings.test_plaid_settings.TestPlaidSettings)
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Traceback (most recent call last):
  File "/home/vishal/digi_frappe_sprint/apps/erpnext/erpnext/erpnext_integrations/doctype/plaid_settings/test_plaid_settings.py", line 162, in test_default_bank_account
    self.assertRaises(frappe.exceptions.ValidationError, add_bank_accounts, response=bank_accounts, bank=bank, company=company)
AssertionError: ValidationError not raised by add_bank_accounts

Fixes::
1.discount gaurd function in selling controller tries to make sales return value to zero but it should be negative
2. removed test_default_bank_account becuase we added functionality in plaid setting which is not needed default bank account if no account found it will create one.
Expected Behaviour:

1. test_payment_against_negative_sales_invoice (erpnext.accounts.doctype.payment_entry.test_payment_entry.TestPaymentEntry) ... ok
2. test_accounts_receivable (erpnext.accounts.report.accounts_receivable.test_accounts_receivable.TestAccountsReceivable) ... ok
3. test_credit_note (erpnext.accounts.doctype.sales_invoice.test_sales_invoice.TestSalesInvoice) ... ok
4. test_return_sales_invoice (erpnext.accounts.doctype.sales_invoice.test_sales_invoice.TestSalesInvoice) ... ok
5.  removed test_default_bank_account