os: in TestDirFS only check returned path for unexpected string
The test added in CL 446115 was failing on Plan 9, on which the error
returned by a failed open includes the path that failed.
Change-Id: If27222596c3cb0366a030bb49ae41c4c869c3db2
Reviewed-on: https://go-review.googlesource.com/c/go/+/446641 Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>