internal: fix flaky context test
internal: fix flaky context test

Change-Id: I877fbcfdde33559baabbda2c275164dd98256892
Reviewed-on: https://go-review.googlesource.com/85196
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/oauth2