// Check that ReadFile works if present.
	if fsys, ok := t.fsys.(fs.ReadFileFS); ok {
		data2, err := fsys.ReadFile(file)
		if err != nil {
			t.errorf("%s: fsys.ReadFile: %w", file, err)
			return
		}
		t.checkFileRead(file, "ReadAll vs fsys.ReadFile", data, data2)

		// Modify the data and check it again. Modifying the
		// returned byte slice should not affect the next call.
		for i := range data2 {
			data2[i]++
		}