cmd/go/internal/fsys: improve initOverlay test helper

cmd/go/internal/fsys: improve initOverlay test helper

1. Remove the use of fmt.Sprintf from t.Fatal.

2. Check for errors from initFromJSON.

3. Move 'overlay=0' to a separate cleanup function, for clarity.

Change-Id: I4d0daad248e8f26b6f159b4cc7e77fd60dc1ed98
Reviewed-on: https://go-review.googlesource.com/c/go/+/527700
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>