Retrieve start time from remote machine, use compat attrib functions

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.