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 da4bc5c927)