Fix using background commands in 'eshell-command'

Fix using background commands in 'eshell-command'

This regressed due to the patch for bug#53715, which changed how
Eshell pipelines return the processes in the pipeline (bug#62556).

* lisp/eshell/esh-cmd.el (eshell-parse-command): When creating
background commands, wrap the process(es) in a cons cell whose CAR is
':eshell-background'.  This lets us use fewer heuristics...
(eshell-eval-command): ... here.  Additionally, keep the result and
the incomplete delimiter separate.

* lisp/eshell/eshell.el (eshell-command): Check ':eshell-background'
and use a more-robust method for setting the output target.

* test/lisp/eshell/eshell-tests.el (eshell-test/eshell-command/simple)
(eshell-test/eshell-command/pipeline)
(eshell-test/eshell-command/background)
(eshell-test/eshell-command/background-pipeline): New tests.