https://go.dev/wiki/CodeReviewComments#useful-test-failures
(Previously it was using subtests with names including the input, but
once those went away, there was no context left)
Updates #14169
Change-Id: Ib217028183a3d001fe4aee58f2edb746b7b3aa88
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>