Wait for all processes in a pipeline before resuming an Eshell command

Wait for all processes in a pipeline before resuming an Eshell command

Previously, we only waited until the tail process was finished, but
now, we wait for all of them.  This is more consistent with other
shells, and prevents some cases of a process's output coming *after*
we continued past its pipeline.

* lisp/eshell/esh-cmd.el (eshell-resume-command): Simplify
conditionals, and check that all the foreground processes are dead
before resuming Eshell command.

* test/lisp/eshell/esh-cmd-tests.el
(esh-cmd-test/pipeline-wait/multi-proc): New test.