lib/fs, lib/model: Rewrite RecvOnly tests (#6318)

During some other work I discovered these tests weren't great, so I've
rewritten them to be a little better. The real changes here are:

- Don't play games with not starting the folder and such, and don't
  construct a fake folder instance -- just use the one the model has. The
  folder starts and scans but the folder contents are empty at this point
  so that's fine.

- Use a fakefs instead of a temp dir.

- To support the above, implement a fakefs option `?content=true` to
  make the fakefs actually retain written content. Use sparingly,
  obviously, but it means the fakefs can usually be used instead of an
  on disk real directory.

lib/model/testutils_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/rand"
 )
 
 var (
@@ -86,6 +87,13 @@ func testFolderConfig(path string) config.FolderConfiguration {
 	return cfg
 }
 
+func testFolderConfigFake() config.FolderConfiguration {
+	cfg := config.NewFolderConfiguration(myID, "default", "default", fs.FilesystemTypeFake, rand.String(32)+"?content=true")
+	cfg.FSWatcherEnabled = false
+	cfg.Devices = append(cfg.Devices, config.FolderDeviceConfiguration{DeviceID: device1})
+	return cfg
+}
+
 func setupModelWithConnection() (*model, *fakeConnection, config.FolderConfiguration) {
 	w, fcfg := tmpDefaultWrapper()
 	m, fc := setupModelWithConnectionFromWrapper(w)