Add optional interface DirCacheFlush for making the tests more reliable

This is defined for the users of dircache drive, onedrive, and acd.

This helps fix the DirMove tests on acd.

onedrive/onedrive.go

@@ -635,6 +635,12 @@ func (f *Fs) Purge() error {
 	return f.purgeCheck("", false)
 }
 
+// DirCacheFlush resets the directory cache - used in testing as an
+// optional interface
+func (f *Fs) DirCacheFlush() {
+	f.dirCache.ResetRoot()
+}
+
 // Hashes returns the supported hash sets.
 func (f *Fs) Hashes() fs.HashSet {
 	return fs.HashSet(fs.HashSHA1)
@@ -960,6 +966,7 @@ var (
 	_ fs.Copier = (*Fs)(nil)
 	// _ fs.Mover    = (*Fs)(nil)
 	// _ fs.DirMover = (*Fs)(nil)
-	_ fs.Object    = (*Object)(nil)
-	_ fs.MimeTyper = &Object{}
+	_ fs.DirCacheFlusher = (*Fs)(nil)
+	_ fs.Object          = (*Object)(nil)
+	_ fs.MimeTyper       = &Object{}
 )