testing/fstest: allow specifying file for "." in MapFS

Prior to this commit, specifying a file for "." in MapFS
created an invalid fs.FS and caused infinite recursion in fs.WalkDir.

Fixes #46776

Change-Id: Ia9e4ae1125355a74dba9ee6b36451b7fda75a862
Reviewed-on: https://go-review.googlesource.com/c/go/+/328409


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>