bug#28319: emacsclient tests interfere with running Emacs, hang

[Glenn Morris]

Package: emacs
Version: 26.0.50

The emacsclient tests added in 98f01a1 fail on hydra, ref eg

  https://hydra.nixos.org/build/59905042
  (wrong-type-argument number-or-marker-p "Profiling timer expired")

On my own desktop machine, they also fail, and interfere with my normal
editing Emacs:

Have an Emacs session open, with an active server.
Run make lib-src/emacsclient-tests
This pops up a "foo" buffer in my desktop Emacs.
The tests now hang and do not return.
The emacsclient process is not killed.

The tests should use their own dedicated server, with a timeout.