internal: use t.Errorf in test output with formatting directives
internal: use t.Errorf in test output with formatting directives

Change-Id: Ic12698e35565201310235b439558ae57e3813ae6
Reviewed-on: https://go-review.googlesource.com/51950
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/text