tests: loopback: check that files create after OPENDIR show up in READDIR

Without patch "nodefs: delay directory read till ReadDir":

	go-fuse/fuse/test$ go test
	13:56:23.932743 Unimplemented opcode INTERRUPT
	13:56:23.932902 writer: Write/Writev failed, err: 2=no such file or directory. opcode: INTERRUPT
	--- FAIL: TestReaddir (0.02s)
	    loopback_test.go:578: Wrong number of directory entries: want=2 have=1
	FAIL
	exit status 1
	FAIL	github.com/hanwen/go-fuse/fuse/test	1.970s

With patch "nodefs: delay directory read till ReadDir":

	go-fuse/fuse/test$ go test
	13:57:03.848021 Unimplemented opcode INTERRUPT
	13:57:03.848159 writer: Write/Writev failed, err: 2=no such file or directory. opcode: INTERRUPT
	PASS
	ok  	github.com/hanwen/go-fuse/fuse/test	1.960s

https://github.com/hanwen/go-fuse/issues/252

fuse/test/loopback_test.go

@@ -25,9 +25,13 @@ import (
 )
 
 type testCase struct {
+	// Per-testcase temporary directory, usually in /tmp, named something like
+	// "$TESTNAME.123456".
 	tmpDir string
-	orig   string
-	mnt    string
+	// Backing directory. Lives in tmpDir.
+	orig string
+	// Mountpoint. Lives in tmpDir.
+	mnt string
 
 	mountFile   string
 	mountSubdir string
@@ -559,7 +563,6 @@ func TestReaddir(t *testing.T) {
 	defer tc.Cleanup()
 
 	contents := []byte{1, 2, 3}
-	tc.WriteFile(tc.origFile, []byte(contents), 0700)
 	tc.Mkdir(tc.origSubdir, 0777)
 
 	dir, err := os.Open(tc.mnt)
@@ -568,6 +571,10 @@ func TestReaddir(t *testing.T) {
 	}
 	defer dir.Close()
 
+	// READDIR should show "hello.txt" even if it is created after the OPENDIR.
+	// https://github.com/hanwen/go-fuse/issues/252
+	tc.WriteFile(tc.origFile, []byte(contents), 0700)
+
 	infos, err := dir.Readdir(10)
 	if err != nil {
 		t.Fatalf("Readdir failed: %v", err)
@@ -578,7 +585,7 @@ func TestReaddir(t *testing.T) {
 		"subdir":    true,
 	}
 	if len(wanted) != len(infos) {
-		t.Errorf("Length mismatch %v", infos)
+		t.Errorf("Wrong number of directory entries: want=%d have=%d", len(wanted), len(infos))
 	} else {
 		for _, v := range infos {
 			_, ok := wanted[v.Name()]