Don't add an extraneous slash in remote PATH list in Eshell

Don't add an extraneous slash in remote PATH list in Eshell

Previously, in a remote directory, '(eshell-get-path)' would return a
list of strings like "/ssh:localhost://usr/bin".  While that shouldn't
break most things, it's not strictly correct either.  See bug#65551.

* lisp/eshell/esh-util.el (eshell-get-path): Use 'concat' instead of
'file-name-concat'.

* test/lisp/eshell/esh-util-tests.el: Require 'tramp' and
'eshell-tests-helpers'.
(esh-util-test/path/get, eshell-util-test/path/get-remote): New tests.