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

[vivekverma-arista]

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

[vivekverma-arista]

raised #373 for this