Adds a new Context method to testing.T, that returns a context, that is
canceled before the end of its test function.

Fixes #36532.

Change-Id: I9315ad4dad25529d0b5be809e2d9db4e7528b5f2
GitHub-Last-Rev: 1c3fd6c4d8a9cc68a61f2df284d04d3d080216be
GitHub-Pull-Request: golang/go#68828
Reviewed-on: https://go-review.googlesource.com/c/go/+/603959


Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>