selftests: drv-net: Fix tolerance calculation in devlink_rate_tc_bw.py

selftests: drv-net: Fix tolerance calculation in devlink_rate_tc_bw.py

Currently, tolerance is computed against the TC’s expected percentage,
making TC3 (20%) validation overly strict and TC4 (80%) overly loose.

Update BandwidthValidator to take a dict of shares and compute bounds
relative to the overall total, so that all shares are validated
consistently.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20251130091938.4109055-7-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>