tests: drop "-z" from fusermount to catch forgotten fds

macos does not have lazy unmount, so let's not use it
on linux either.
If the unmount fails, run "lsof" to find the open file.

Also fix the first bug we found this way.

tests/matrix/matrix_test.go

@@ -796,6 +796,7 @@ func TestUtimesNanoFd(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer f.Close()
 	procPath := fmt.Sprintf("/proc/self/fd/%d", f.Fd())
 	doTestUtimesNano(t, procPath)
 }

tests/test_helpers/helpers.go

@@ -195,14 +195,18 @@ func MountOrFatal(t *testing.T, c string, p string, extraArgs ...string) {
 func UnmountPanic(dir string) {
 	err := UnmountErr(dir)
 	if err != nil {
-		fmt.Println(err)
-		panic(err)
+		fmt.Printf("UnmountPanic: %v. Running lsof %s\n", err, dir)
+		cmd := exec.Command("lsof", dir)
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		cmd.Run()
+		panic("UnmountPanic: unmount failed: " + err.Error())
 	}
 }
 
 // UnmountErr tries to unmount "dir" and returns the resulting error.
 func UnmountErr(dir string) error {
-	cmd := exec.Command(UnmountScript, "-u", "-z", dir)
+	cmd := exec.Command(UnmountScript, "-u", dir)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	return cmd.Run()