Stabilize channels variant of erc-reuse-buffers test

Stabilize channels variant of erc-reuse-buffers test

* lisp/erc/erc-networks.el (erc-networks--id-sort-buffers): Use
`buffer-local-value' instead of `with-current-buffer'.

* test/lisp/erc/erc-scenarios-base-reuse-buffers.el
(erc-scenarios-common--base-reuse-buffers-channel-buffers): Wait for
buffers to be created by server-initiated JOINs.
(erc-scenarios-base-reuse-buffers-channel-buffers--disabled): Remove
`:unstable' tag.

* test/lisp/erc-tests.el (erc-ring-previous-command): Remove
unnecessary `goto-char'.