fstest: shut down test remotes when finished with them

Remotes made by RandomRemote aren't in the fs cache so they were never
shut down, leaving any pooled connections open until they timed out.

On TestFTPProftpd this used up the server's 30 connection limit part way
through the fs/operations tests, making all the following tests fail
with EOF when connecting.

fstest/fstest.go

@@ -476,6 +476,14 @@ func RandomRemote() (fs.Fs, string, func(), error) {
 				fs.Logf(nil, "Failed to purge %v: %v", parentRemote, err)
 			}
 		}
+		// The remote isn't in the fs cache so release its resources
+		// (e.g. pooled connections) here rather than letting them
+		// pile up until they time out.
+		if do := remote.Features().Shutdown; do != nil {
+			if err := do(context.Background()); err != nil {
+				fs.Logf(nil, "Failed to shutdown %v: %v", remote, err)
+			}
+		}
 	}
 
 	return remote, remoteName, finalise, nil