Improve timer handling when Tramp accepts output

Improve timer handling when Tramp accepts output

* lisp/net/tramp-compat.el: Avoid compiler warning.

* lisp/net/tramp-sh.el (tramp-sh-file-name-handler):
Remove lock machinery.

* lisp/net/tramp.el (tramp-locked, tramp-locker): Move up.
(tramp-file-name-handler): Add lock machinery from
`tramp-sh-file-name-handler'.  Allow timers to run.
(tramp-accept-process-output): Remove nasty workaround.
Suppress timers.

* test/lisp/net/tramp-tests.el (shell-command-sentinel):
Suppress run in tests.
(tramp--instrument-test-case-p): New defvar.
(tramp--instrument-test-case): Use it in order to allow nested calls.
(tramp--test-message, tramp--test-backtrace): New defsubst,
will be used for occasional test instrumentation.
(tramp-test00-availability, tramp-test31-vc-registered): Use them.
(tramp-test28-shell-command)
(tramp--test-shell-command-to-string-asynchronously): Suppress
nasty messages.  Don't overwrite sentinel.
(tramp-test36-asynchronous-requests): Rewrite major parts.
Expect :passed.