[test] fix `usbdev_deep_disconnect_test` flakiness on cw310

> @timothytrippel The failed test: `usbdev_deep_disconnect_test` in the CI should be unrelated to this PR since this PR only adds new e2e tests and doesn't modify the existing usb driver code or that failed usb test. I can observe that `usbdev_deep_disconnect_test` also failed in the CI from a different PR: https://github.com/lowRISC/opentitan/actions/runs/20354991553/job/58596885696?pr=28976#step:6:35068

This test is known to be broken/super flaky on the CW310. Unfortunately, the proper fix for that would be make it work on the CW340 because the CW310 does not support proper USB VBUS management which requires us to do some hacks. I have some WIP code for the CW340 but it is less of a priority (it works on real silicon which is where the most important).

_Originally posted by @pamaury in https://github.com/lowRISC/opentitan/issues/28789#issuecomment-3681527678_

Start by reproducing `usbdev_deep_disconnect_test` on the CW310 in CI and reviewing the USB VBUS-management behavior described in the issue. Check the available WIP CW340 work as context. Done means the test no longer flakes, with the required board-specific behavior understood and validated.

Written by the indexing model from the issue text.