Always perform Eshell process cleanup runs in the Eshell buffer

Always perform Eshell process cleanup runs in the Eshell buffer

Previously, some code executed in a timer, which could execute in the
wrong buffer, leading to a hang.

* lisp/eshell/esh-proc.el (eshell-sentinel): Use 'with-current-buffer'
in the timer function.

* test/lisp/eshell/esh-proc-tests.el (eshell-test-value): New variable.
(esh-proc-test/sentinel/change-buffer): New test.

(cherry picked from commit da4bc5c9274a6705501e24fb8f2984f5bf5fe099)