Incorrect calculation of time duration for which traffic blackholed in case of `test_bgp_outbound_ungraceful_restart.py`

The downtime calculated here uses: https://github.com/Azure/sonic-mgmt.msft/blob/202405/tests/snappi_tests/multidut/bgp/files/bgp_outbound_helper.py#L2041-L2044 statistics queried before issuing kernel panic at this point https://github.com/Azure/sonic-mgmt.msft/blob/202405/tests/snappi_tests/multidut/bgp/files/bgp_outbound_helper.py#L2018-L2019

This leads to an underestimated lossy period.

At all the other places this is being done correctly after the disruptive event. eg in the link flap test

https://github.com/Azure/sonic-mgmt.msft/blob/202405/tests/snappi_tests/multidut/bgp/files/bgp_outbound_helper.py#L1235

Start in tests/snappi_tests/multidut/bgp/files/bgp_outbound_helper.py at lines 2018-2019 and 2041-2044, then compare the timing flow with line 1235 used by the link flap test. Verify that the traffic statistics used for downtime are queried after the kernel panic, and confirm that test_bgp_outbound_ungraceful_restart.py reports the corrected lossy period.

Written by the indexing model from the issue text.