context: mark testDeadline as a testing helper method
context: mark testDeadline as a testing helper method

Change-Id: Ie6fc3e9789aea6e5949e66186db6f2b071b6fdff
Reviewed-on: https://go-review.googlesource.com/c/go/+/204037
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go