Backport: Minor changes in tramp-tests.el

Backport: Minor changes in tramp-tests.el

* test/lisp/net/tramp-tests.el (tramp--test-shell-file-name):
Use connection-local value.
(tramp--test-shell-command-switch): New defun.
(tramp-test28-process-file)
(tramp-test34-explicit-shell-file-name): Use it.
(tramp--test-supports-processes-p): Simplify.
(tramp--test-check-files): Use `tramp-compat-seq-keep'.
(tramp-test45-asynchronous-requests): Don't let-bind `shell-file-name'.
(tramp-test45-asynchronous-requests): Adjust timer.
(tramp-test45-asynchronous-requests): Add another test message.

(cherry picked from commit c95caade15d295fa6cc31d337028faa15142b388)