Move ERC's buffer-display tests to separate file

Move ERC's buffer-display tests to separate file

* test/lisp/erc/erc-scenarios-base-buffer-display.el: New file.
* test/lisp/erc/erc-scenarios-base-reconnect.el
(erc-scenarios-common--base-reconnect-options,
erc-scenarios-base-reconnect-options--buffer,
erc-scenarios-base-reconnect-options--default): Move to new file and
rename.  (Bug#62833)
* test/lisp/erc/resources/erc-d/erc-d-tests.el (erc-d-run-linger):
Lengthen timeout.
* test/lisp/erc/resources/erc-d/erc-d.el (erc-d--m): Ensure buffer is
live before inserting.