Be more precise about navigating forward/backward through Eshell prompts

Be more precise about navigating forward/backward through Eshell prompts

* lisp/eshell/em-prompt.el (eshell-next-prompt): Make N optional.
When navigating, first move to the end of the prompt.  This makes the
subsequent navigation more predictable.
(eshell-previous-prompt): Mane N optional.

* test/lisp/eshell/em-prompt-tests.el
(em-prompt-test--with-multiline): Move to "Code" section.
(em-prompt-test/next-previous-prompt-with): Rename to...
(em-prompt-test/next-previous-prompt-1): ... this, and add additional
test cases.  Update callers.
(em-prompt-test/forward-backward-matching-input-with): Rename to...
(em-prompt-test/forward-backward-matching-input-1): ... this, and
improve existing test cases.