Replace erc--get-inserted-msg-bounds with functions

Replace erc--get-inserted-msg-bounds with functions

* lisp/erc/erc-fill.el (erc-fill--wrap-rejigger-region): Call
`erc--get-inserted-msg-end' instead of `erc--get-inserted-msg-bounds'
with `end' arg.
* lisp/erc/erc-truncate.el (erc-truncate-buffer-to-size): Call
`erc--get-inserted-msg-beg' with `point' instead of
`erc--get-inserted-msg-bounds' with `beg' arg.
* lisp/erc/erc.el (erc--get-inserted-msg-beg-at,
erc--get-inserted-msg-end-at): New macros.
(erc--get-inserted-msg-beg, erc--get-inserted-msg-end): New functions.
(erc--get-inserted-msg-bounds): Convert to function with different
signature, and refactor.  This was introduced for the yet unreleased
5.6 (Emacs 30) by bug#60936.
(erc--get-inserted-msg-prop): Use `erc--get-inserted-msg-beg' instead
of `erc--get-inserted-msg-bounds' with `beg' arg.
(erc--delete-inserted-message): Update call site of
`erc--get-inserted-msg-bounds' to match new signature.
(erc-cmd-CLEAR): Use `erc--get-inserted-msg-beg' instead of
`erc--get-inserted-msg-bounds' with `beg' arg.
* test/lisp/erc/erc-scenarios-match.el
(erc-scenarios-match--stamp-right-fools-invisible): Call
`erc--get-inserted-msg-end' instead of `erc--get-inserted-msg-bounds'.
* test/lisp/erc/erc-tests.el (erc-tests--get-inserted-msg-setup,
erc-tests--assert-get-inserted-msg,
erc-tests--assert-get-inserted-msg/basic,
erc-tests--assert-get-inserted-msg/stamp,
erc-tests--assert-get-inserted-msg-readonly-with): New helper
functions.
(erc--get-inserted-msg-beg/basic, erc--get-inserted-msg-beg/readonly,
erc--get-inserted-msg-beg/stamp,
erc--get-inserted-msg-beg/readonly/stamp,
erc--get-inserted-msg-end/basic, erc--get-inserted-msg-end/stamp,
erc--get-inserted-msg-end/readonly,
erc--get-inserted-msg-end/readonly/stamp,
erc--get-inserted-msg-bounds/readonly,
erc--get-inserted-msg-bounds/stamp,
erc--get-inserted-msg-bounds/readonly/stamp): New tests.
(erc--get-inserted-msg-bounds, erc--get-inserted-msg-bounds/basic):
Move `beg' and `end' cases to separate test variants and rename former
to latter.