Simplify Eshell handle functions and add tests/documentation

Simplify Eshell handle functions and add tests/documentation

* lisp/eshell/esh-arg.el (eshell-parse-argument-hook): Explain how to
use 'eshell-finish-arg'.

* lisp/eshell/esh-io.el (eshell-create-handles): Only call
'eshell-get-target' for stderr if necessary.
(eshell-protect-handles): Use 'dotimes'.
(eshell-set-output-handle): Pass HANDLES and fix an edge case with
setting a duplicate TARGET.

* test/lisp/eshell/eshell-tests-helpers.el (eshell-with-temp-buffer):
New macro.

* test/lisp/eshell/esh-cmd-tests.el (esh-cmd-test/quoted-lisp-form)
(esh-cmd-test/backquoted-lisp-form)
(esh-cmd-test/backquoted-lisp-form/splice): New tests.

* test/lisp/eshell/eshell-tests.el (eshell-test/redirect-buffer)
(eshell-test/redirect-buffer-escaped): Move to...
* test/lisp/eshell/esh-io-tests.el: ... here, and add other I/O tests.

* doc/misc/eshell.texi (Arguments): Add documentation for special
argument types.
(Input/Output): Expand documentation for redirection and pipelines.