Rework eshell-match-result for testing asynchronous commands

Rework eshell-match-result for testing asynchronous commands

When using eshell-match-result via eshell-command-result-p to examine
the output of asynchronous Eshell commands, a newly emitted prompt is
included in the text against which the regexp is matched.  This makes
it awkward to match against the whole output; for example, to check
whether it is empty.  Rework the function to exclude the prompt.

* test/lisp/eshell/eshell-tests-helpers.el (eshell-match-result):
Exclude any newly emitted prompt from the text against which the
regexp is matched.  Additionally, the function no longer moves point.
* test/lisp/eshell/eshell-tests.el (eshell-test/flush-output): Update
and simplify test given how eshell-match-result no longer moves point.