Add 'eshell-command-result-equal' with an ERT explainer

Add 'eshell-command-result-equal' with an ERT explainer

* test/lisp/eshell/eshell-tests-helpers.el
(eshell-command-result--equal, eshell-command-result--equal-explainer)
(eshell-command-result-equal): New functions.

* test/lisp/eshell/em-basic-tests.el
* test/lisp/eshell/em-dirs-tests.el
* test/lisp/eshell/esh-cmd-tests.el
* test/lisp/eshell/esh-proc-tests.el
* test/lisp/eshell/esh-var-tests.el
* test/lisp/eshell/eshell-tests.el: Use 'eshell-command-result-equal'.