bug#36940: tests slowness and failure after recent Tramp changes

[Paul Eggert]

Michael Albinus wrote:

> A more precise test would be to retrieve the ert test start time, as taken
> from the remote machine. But I don't know of a general function doing this.

Can we do the equivalent of 'touch nonce-file' on the remote machine, and then 
retrieve the nonce-file's timestamp? That should be a reasonably-reliable way of 
getting a remote timestamp at the start of the test.