testing/fstest: new package for testing file system code

testing/fstest: new package for testing file system code

This change adds basic test helpers for file system code.

The type MapFS is a simple map-based file system for use when
exercising general file system code.

The func TestFS tests a file system implementation.

For #41190.

Change-Id: I5a2036f57e733915ad508651ad7317749794423c
Reviewed-on: https://go-review.googlesource.com/c/go/+/243910
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>