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

[Glenn Morris]

I suspect it is now unstable. On RHEL 7.6 at d5622eb:

for ((n=1;n<=100;n++)); do
  make lisp/net/tramp-tests 
SELECTOR='tramp-test19-directory-files-and-attributes' 2>&1 | grep -q '[^0] 
unexpected' && let fail++
done

echo $fail

returned 7 failures for me.