Fix regression in erc-keep-place-indicator-mode

Fix regression in erc-keep-place-indicator-mode

* lisp/erc/erc-goodies.el (erc-keep-place): Revert portion of
65735efdca0 "Improve multi-window erc-keep-place-indicator-mode" that
skipped modifying `window-prev-buffers' when the indicator's minor
mode was enabled.
* test/lisp/erc/erc-scenarios-keep-place-indicator.el: Modify test to
assert that point's place is preserved after switching away from a
buffer with the indicator enabled.
* test/lisp/erc/resources/keep-place/follow.eld: Update.
This feature was originally introduced by bug#59943.