testing/fstest: Glob generation fails on minus in directory name

[cugu]

fstest.TestFS generates a glob pattern (testfs.go#L294), but generates an incorrect one when the directory contains a minus.

What version of Go are you using (go version)?

$ go version
go version go1.16 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

func TestFS(t *testing.T) {
	fsys := fstest.MapFS{
		"a-b/a": &fstest.MapFile{},
	}
	if err := fstest.TestFS(fsys, "a-b/a"); err != nil {
		t.Error(err)
	}
}

go test ./...

What did you expect to see?

ok  	github.com/foo/bar	4.953s

What did you see instead?

go test ./...
--- FAIL: TestFS (0.00s)
    fs_test.go:16: TestFS found errors:
        a-b: Glob(`a[-][b-b]/*a*`): syntax error in pattern
FAIL
FAIL	github.com/foo/bar	4.052s
FAIL

CC @rsc

[gopherbot]

Change https://golang.org/cl/294869 mentions this issue: testing/fstest: treat dash specially when building glob