Fix redirecting Eshell output to symbols in some places

Fix redirecting Eshell output to symbols in some places

Do not merge to master.

* lisp/eshell/esh-io.el (eshell-output-object-to-target): Don't require
TARGET to be bound.

* lisp/eshell/em-script.el (eshell-execute-file): Quote the output/error
targets.

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

* 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.