Retrieve start time from remote machine, use compat attrib functions

* lisp/net/tramp-compat.el (tramp-compat-file-attribute-access-time)
(tramp-compat-file-attribute-status-change-time): New defaliases.

* test/lisp/net/tramp-tests.el (tramp--test-start-time): New defvar.
(tramp--test-file-attributes-equal-p)
(tramp-test19-directory-files-and-attributes): Use it.
(tramp-test18-file-attributes)
(tramp--test-file-attributes-equal-p, tramp-test20-file-modes)
(tramp-test22-file-times, tramp--test-check-files):
Use `tramp-compat-file-attribute-*' functions.
1 job for master in 52 minutes and 54 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2953
test-all

00:52:54

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1