Cement ordering of essential hook members in ERC

Cement ordering of essential hook members in ERC

* etc/ERC-NEWS: Add new section explaining the pinning of certain hook
members owned by built-in modules to fixed depths.
* lisp/erc/erc-button.el (erc-button-mode, erc-button-enable): Change
hook depth for `erc-button-add-buttons' from 90 to 30.
* lisp/erc/erc-fill.el (erc-fill-mode, erc-fill-enable): Change hook
depth for `erc-fill' from 0 to 40.
* lisp/erc/erc-match.el (erc-match-mode, erc-match-enable): Change
hook depth for `erc-match-message' from 90 to 60.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-enable): Change
hook depth for `erc-add-timestamp' from 90 to 50.
* test/lisp/erc/erc-tests.el
(erc-tests--assert-printed-in-subprocess): Add fixture for testing a
form printed from a subprocess.
(erc--find-mode, erc--essential-hook-ordering): Use helper in existing
and new tests, respectively.  (Bug#60936)