memory: fix incorrect list entries when rooted at subdirectory

Before this change, List would return incorrect directory paths (relative to the
wrong root) if the Fs root pointed to a subdirectory. For example, listing dir
"a/b/c/d" of remote :memory: would work correctly, but listing dir "c/d" of
remote :memory:a/b would not, and would result in "Entry doesn't belong in
directory %q (contains subdir)" errors.

This change fixes the issue and adds a test to detect any other backends that
might have the same issue.

backend/memory/memory.go

@@ -296,7 +296,7 @@ func (f *Fs) list(ctx context.Context, bucket, directory, prefix string, addBuck
 				slash := strings.IndexRune(localPath, '/')
 				if slash >= 0 {
 					// send a directory if have a slash
-					dir := directory + localPath[:slash]
+					dir := strings.TrimPrefix(directory, f.rootDirectory+"/") + localPath[:slash]
 					if addBucket {
 						dir = path.Join(bucket, dir)
 					}