Refactor marker initialization in erc-open

Refactor marker initialization in erc-open

* lisp/erc/erc.el (erc--initialize-markers): New helper to ensure
prompt and its associated markers are set up correctly.
(erc-open): When determining whether a session is a logical
continuation, leverage the work already performed by the
`erc-networks' library to that effect.  Its verdicts are based on
network context and thus reliable even when a user dials anew from an
entry-point, which is not a simple reconnection because the user
expects a clean slate for everything except an existing buffer's
messages, meaning `erc--server-reconnecting' will be nil and
local-module state variables need resetting.  Also remove the check
for `erc-reuse-buffers' and instead trust that `erc-get-buffer-create'
always does the right thing.  Replace all code involving marker and
prompt setup by deferring to a new helper, `erc--initialize markers'.
* test/lisp/erc/erc-scenarios-base-local-module-modes.el: New file.
* test/lisp/erc/erc-scenarios-base-local-modules.el
(erc-scenarios-base-local-modules--mode-persistence): Move test to
separate file to help with parallel "-j" runs.
* test/lisp/erc/erc-tests.el (erc-tests--send-prep): Replace
redundant prompt-setup code.
(erc--initialize-markers): New test.  (Bug#60936.)