Fix using background commands in 'eshell-command'

Fix using background commands in 'eshell-command'

Do not merge to master.

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-eval-command): Allow process-pairs.

* 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.