Allow erc-reuse-frames to favor connections

Allow erc-reuse-frames to favor connections

* lisp/erc/erc.el (erc-reuse-frames): Add alternate value to favor
existing frames already displaying buffers from the same connection.
(erc--setup-buffer-first-window, erc--display-buffer-use-some-frame):
Add helpers to support 'display' variant of `erc-resuse-frames'
* test/lisp/erc/erc-tests.el (erc-tests--run-in-term,
erc-tests--servars, erc-reuse-frames, erc-tests--erc-reuse-frames,
erc-tests--erc-reuse-frames--t, erc-resuse-frames--t,
erc-tests--erc-reuse-frames--displayed-single,
erc-reuse-frames--displayed-single, erc-tests--assert-server-split,
erc-tests--erc-reuse-frames--displayed-double,
erc-reuse-frames--displayed-double,
erc-tests--erc-reuse-frames--displayed-full,
erc-reuse-frames--displayed-full): Add test case and supporting
fixtures.  (Bug#55540.)