Fix Eshell tests on MS-Windows

Fix Eshell tests on MS-Windows

* lisp/eshell/esh-util.el (eshell-get-path): Don't add "." if it
is already there.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/path-var/preserve-across-hosts): Skip on MS-Windows.
(esh-var-test/path-var/set, esh-var-test/path-var/set-locally):
Quote the PATH value, for MS-Windows.
* test/lisp/eshell/esh-util-tests.el (esh-util-test/path/get): No
need to add ".": it is already done by 'eshell-get-path'.
* test/lisp/eshell/esh-proc-tests.el
(esh-proc-test/kill-pipeline): Accept empty string as valid
output.
(esh-proc-test/sigpipe-exits-process): Skip on MS-Windows: no
SIGPIPE.
(esh-proc-test/emacs-command): Quote correctly for MS-Windows.
* test/lisp/eshell/em-unix-tests.el
(em-unix-test/compile/interactive): Fix test on MS-Windows.
* test/lisp/eshell/em-script-tests.el (em-script-test/batch-file):
Skip on MS-Windows.
* test/lisp/eshell/eshell-tests-helpers.el
(eshell-command-result--equal): Compare strings (file names)
case-insensitively on MS-Windows.

(cherry picked from commit f869f1ffc2ef0e126e633553e6b4c38bee90f7f8)