Use rls in travis ci and add actual tests

Use rls in travis ci and add actual tests

Also run a hook when connected

* eglot-tests.el (eglot--with-dirs-and-files)
(eglot--make-file-or-dirs, eglot--call-with-dirs-and-files)
(eglot--find-file-noselect): New helpers.
(auto-detect-running-server, auto-reconnect): New actual tests.

* eglot.el (eglot-connect): Run hook when connected
(eglot-connect-hook): New variable

* .travis.yml: Use rust stable and install rls

* README.md: Update mention of automated tests