Fix handling of output handles in nested Eshell forms

Fix handling of output handles in nested Eshell forms

Previously, the output handles in nested forms would be reset to the
default, leading to wrong behavior for commands like

  {echo a; echo b} > file

"b" would be written to "file" as expected, but "a" would go to
standard output (bug#59545).

* lisp/eshell/esh-cmd.el (eshell-parse-command): Use
'eshell-with-copied-handles' for each statement within the whole
Eshell command.

* test/lisp/eshell/esh-io-tests.el (esh-io-test/redirect-subcommands)
(esh-io-test/redirect-subcommands/override)
(esh-io-test/redirect-subcommands/interpolated): New tests.

* test/lisp/eshell/em-script-tests.el
(em-script-test/source-script/redirect)
(em-script-test/source-script/redirect/dev-null): New tests.
(em-script-test/source-script, em-script-test/source-script/arg-vars)
(em-script-test/source-script/all-args-var): Tweak names/docstrings.

* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-tests--deftest):
Skip over the newly-added 'eshell-with-copied-handles' form when
checking the parse results.

* test/lisp/eshell/em-tramp-tests.el (em-tramp-test/su-default)
(em-tramp-test/su-user, em-tramp-test/su-login)
(em-tramp-test/sudo-shell, em-tramp-test/sudo-user-shell)
(em-tramp-test/doas-shell, em-tramp-test/doas-user-shell): Update
expected command forms.