embed/internal/embedtest: test openFile.ReadAt with non-zero offset

embed/internal/embedtest: test openFile.ReadAt with non-zero offset

Also fix log messages to report expected values in the standard "got foo,
want bar" format.

Change-Id: I6a9fd4abe1f86c2651c72c2bf7ac4588028e5923
Reviewed-on: https://go-review.googlesource.com/c/go/+/484715
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>