Make erc-fill-wrap depend on scrolltobottom

Make erc-fill-wrap depend on scrolltobottom

* lisp/erc/erc-fill.el (erc-fill-mode): Add reference to
`erc-fill-wrap-mode' in doc string.
(erc--fill-wrap-scrolltobottom-exempt-p): New variable to allow tests
involving `fill-wrap' to opt out of having to enable `scrolltobottom'.
(erc-fill--wrap-ensure-dependencies): Warn and enable
`erc-scrolltobottom-mode' if necessary.
(erc-fill-wrap-mode): Mention workaround for automatically enabling
`scrolltobottom'.
* test/lisp/erc/erc-fill-tests.el (erc-fill-tests--wrap-populate):
Exempt tests from `scrolltobottom' dependency.
* test/lisp/erc/resources/erc-scenarios-common.el: Load `erc-fill'
when compiling.
(erc-scenarios-common--print-trace): Exempt tests using `fill-wrap'
from the `scrolltobottom' dependency by making
`erc--fill-wrap-scrolltobottom-exempt-p' non-nil during test runs.
(Bug#60936)