fstest: fix slice bounds out of range error when using -remotes local

Before this change, TestIntegration/FsName could fail with "slice bounds out of
range [:-1]" when run with -remotes local.

It also caused issues with
'^TestGitAnnexFstestBackendCases$/^(TransferStorePathWithInteriorWhitespace|TransferStoreRelative)$'.

This change fixes the issue by accepting either "" or "local" to indicate the
local remote.

fstest/fstest.go

@@ -65,6 +65,9 @@ func Initialise() {
 	if envConfig := os.Getenv("RCLONE_CONFIG"); envConfig != "" {
 		_ = config.SetConfigPath(envConfig)
 	}
+	if *RemoteName == "local" {
+		*RemoteName = ""
+	}
 	configfile.Install()
 	accounting.Start(ctx)
 	if *Verbose {