Make auth-source erc-services tests more readable

Make auth-source erc-services tests more readable

* lisp/erc/erc-common.el: (erc-with-server-buffer): Modify slightly to
use `buffer-local-value' when possible.
* test/lisp/erc/erc-services-tests.el
(erc-services-tests--auth-source-plstore-standard-entries,
erc-services-tests--auth-source-plstore-standard-secrets): Remove
unused variables.
(erc-services-tests--auth-source-plstore-standard-announced): Add new
var to hold common plstore document.
(erc--auth-source-search--plstore-standard,
erc--auth-source-search--plstore-announced,
erc--auth-source-search--plstore-overrides): Use string literals for
text-document content.
(erc-services-tests--auth-source-json-standard-entries): Remove unused
variable.
(erc-services-tests--auth-source-json-standard-announced): Add new
variable.
(erc--auth-source-search--json-standard,
erc--auth-source-search--json-announced,
erc--auth-source-search--json-overrides): Use string literals for
text-document content.
(erc-services-tests--secrets-search-items): Add new helper function.
(erc--auth-source-search--secrets-standard,
erc--auth-source-search--secrets-announced,
erc--auth-source-search--secrets-overrides): Use helper to mock
`secrets-search-items' instead of misleading lambda.
* lisp/erc/erc-tests.el (erc-with-server-buffer): Add test.