Use 'eshell-with-handles' in a few more places

Use 'eshell-with-handles' in a few more places

* lisp/eshell/em-alias.el (eshell-write-aliases-list):
* lisp/eshell/em-script.el (eshell-batch-file):
* lisp/eshell/esh-cmd.el (eshell-command-to-value):
* lisp/eshell/eshell.el (eshell-command): Use 'eshell-with-handles'.

* test/lisp/eshell/esh-io-tests.el (eshell-test-file-string): Move to...
* test/lisp/eshell/eshell-tests-helpers.el (eshell-test-file-string):
... here.

* test/lisp/eshell/eshell-tests.el
(eshell-test/eshell-command/output-file):
* test/lisp/eshell/em-script-tests.el
(em-script-test/execute-file/output-file): New tests.

(cherry picked from commit 5c68545a936ab42df90c8498ca77207a5e6aff1f)