When a test calls t.Fatal()/t.Fatalf(), only deferred code will execute.
Increment the failure count as part of a deferred call.

Fixes #24412

Change-Id: Ibb154015fcd3d0fb7739718fdda8c9ad22f9e896
Reviewed-on: https://go-review.googlesource.com/101035
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>