Improve robustness of server.el tests

Improve robustness of server.el tests

* lisp/emacs-lisp/ert.el (ert--insert-infos): Allow 'message' to be a
function that is called when inserting the info.
(ert-info): Update docstring to describe using a function for
MESSAGE-FORM.

* lisp/server.el (server-start): Log when the server is starting.

* test/lisp/server-tests.el (server-tests/can-create-frames-p): New
constant.  Use it to skip tests that need to create frames.
(server-tests/start-emacsclient): Rename to...
(server-tests/start-client): ... this, and set the process's buffer.
(server-tests/with-server): Put the server file in a temporary
directory so we don't conflict with real Emacs servers.
(server-tests/with-client): New macro...
(server-tests/server-start/stop-prompt-with-client)
(server-tests/emacsclient/server-edit)
(server-tests/emacsclient/create-frame)
(server-tests/emacsclient/create-frame): ... use it.
(server-tests/server-start/stop-prompt-with-client): Simplify.