testing/fstest,os: clarify racy behavior of TestFS

The testing.TestFS function assumes that the file system it's testing
doesn't change under it. Clarify this in the documentation and fix the
use of os.TestDirFS that's currently susceptible to this race.

Fixes #42637.

Change-Id: Ia7792380726177f8953d150ee87381b66cb01cb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/282452
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/os/os_test.go

@@ -2687,7 +2687,7 @@ func TestOpenFileKeepsPermissions(t *testing.T) {
 }
 
 func TestDirFS(t *testing.T) {
-	if err := fstest.TestFS(DirFS("./signal"), "signal.go", "internal/pty/pty.go"); err != nil {
+	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
 		t.Fatal(err)
 	}
 }

src/testing/fstest/testfs.go

@@ -24,6 +24,7 @@ import (
 // It also checks that the file system contains at least the expected files.
 // As a special case, if no expected files are listed, fsys must be empty.
 // Otherwise, fsys must only contain at least the listed files: it can also contain others.
+// The contents of fsys must not change concurrently with TestFS.
 //
 // If TestFS finds any misbehaviors, it returns an error reporting all of them.
 // The error text spans multiple lines, one per detected misbehavior.