Fix case that $PATH is longer than 4096 chars in Tramp

Fix case that $PATH is longer than 4096 chars in Tramp

* lisp/net/tramp-sh.el (tramp-set-remote-path): Use `tramp-send-command'
directly.

* test/lisp/net/tramp-tests.el (tramp-test-vec): New defconst.
(tramp--test-enabled, tramp--test-message)
(tramp--test-backtrace, tramp-test03-file-name-host-rules)
(tramp-test18-file-attributes, tramp-test20-file-modes)
(tramp-test26-file-name-completion)
(tramp-test33-environment-variables, tramp-test35-remote-path)
(tramp-test36-vc-registered, )
(tramp-test43-asynchronous-requests): Use it.
(tramp-test29-start-file-process, tramp-test30-make-process)
(tramp-test33-environment-variables): Remove Hydra specific result.
(tramp-test35-remote-path): Rewrite partially.