Provide ERT explainer for 'eshell-match-command-output'

Provide ERT explainer for 'eshell-match-command-output'

This was formerly named 'eshell-command-result-p', but "result" isn't
quite the right terminology, since this function specifically tested
the output of a command, not its Lisp result (as
'eshell-command-result' would return).

* test/lisp/eshell/eshell-tests-helpers.el (eshell-insert-command):
Provide a more-complete docstring.
(eshell-match-result): Rename to...
(eshell-match-output): ... this.
(eshell-match-output--explainer): New function.
(eshell-command-result-p): Rename to...
(eshell-match-command-output): ... this.

* test/lisp/eshell/em-alias-tests.el
* test/lisp/eshell/em-dirs-tests.el
* test/lisp/eshell/em-extpipe-tests.el
* test/lisp/eshell/em-script-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-helpers.el
* test/lisp/eshell/eshell-tests.el: Use 'eshell-match-command-output'.