Restore prompt correctly when reconnecting in ERC

Restore prompt correctly when reconnecting in ERC

* lisp/erc/erc.el (erc--initialize-markers): Commit 0d6c8d41ab7 "Use
overlay instead of text prop to hide ERC's prompt" introduced a bug
that caused the prompt to remain hidden upon reconnecting because the
stashed overlay would get clobbered by ERC's major-mode setup.
Binding its old value while unhiding fixes the issue.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Don't permanently set
the default value of `erc-hide-prompt'.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--join-network-id): Add assertions for prompt
hiding and unhiding on reconnect.  (Bug#51082)