internal/http3: use bubbled context in synctest tests

Avoid using T.Context within a synctest bubble. If the Context's
Done channel is created outside the bubble, waiting on it within
the bubble is durably blocking. If it's created within the bubble,
the testing package encounters a panic when closing it after
CL 671960.

Instead, create our own Context within the bubble and cancel it
before the bubble is destroyed.

This will be entirely obviated by synctest.Test, which creates
a testing.T that returns a properly bubbled context.

Change-Id: Iff93c296ccbc1ece8172cb0a60e626ea1bd895ad
Reviewed-on: https://go-review.googlesource.com/c/net/+/675615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>