* test/lisp/erc/erc-tests.el (erc-tests--assert-printed-in-subprocess):
Scan for sentinel before reading.
(erc--find-mode, erc--essential-hook-ordering): Use contrived :result
protocol expected by `erc-tests--assert-printed-in-subprocess'.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-create-subprocess): Divert stderr to messages buffer.