fmt: add tests showing current Scanf space handling

fmt: add tests showing current Scanf space handling

There are no semantic changes here, just tests to establish
the status quo. A followup CL will make some semantic changes,
the (limited) scope of which should be clear from the number of
tests that change.

For #13565.

Change-Id: I960749cf59d4dfe39c324875bcc575096654f883
Reviewed-on: https://go-review.googlesource.com/30610
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>