From the doc comment:

Context returns the context for the current test or benchmark.
The context is cancelled when the test or benchmark finishes.
A goroutine started during a test or benchmark can wait for the
context's Done channel to become readable as a signal that the
test or benchmark is over, so that the goroutine can exit.

Fixes #16221.
Fixes #17552.

Change-Id: I657df946be2c90048cc74615436c77c7d9d1226c
Reviewed-on: https://go-review.googlesource.com/31724Reviewed-by: Rob Pike <r@golang.org>