epg: Add a way to detect gpg1 executable for tests
Fixes bug#23561.
* test/automated/epg-tests.el
(epg-tests-program-alist-for-passphrase-callback): New
constant.
(epg-tests-find-usable-gpg-configuration): New function,
renamed from `epg-tests-gpg-usable'. All callers changed.
(epg-tests-gpg-usable): Remove.
* lisp/epg-config.el (epg-config--program-alist): Factor out
constructor element to...
(epg-config--configuration-constructor-alist): ...here.
(epg-find-configuration): Rename FORCE argument to NO-CACHE,
and add PROGRAM-ALIST argument.