fstest: stop errors in test cleanup changing the global stats

This was causing the concurrent bisync tests to fail every now and again.

fstest/fstest.go

@@ -482,7 +482,9 @@ func RandomRemote() (fs.Fs, string, func(), error) {
 //
 // It logs errors rather than returning them
 func Purge(f fs.Fs) {
-	ctx := context.Background()
+	// Create a stats group here so errors in the cleanup don't
+	// interfere with the global stats.
+	ctx := accounting.WithStatsGroup(context.Background(), "test-cleanup")
 	var err error
 	doFallbackPurge := true
 	if doPurge := f.Features().Purge; doPurge != nil {