Don't test symmetric operations on gpg 2.0 (Bug#23561)

Don't test symmetric operations on gpg 2.0 (Bug#23561)

On the Hydra test machines, which have gpg 2.0, the symmetric tests
fail.
* test/lisp/epg-tests.el (with-epg-tests): Pass REQUIRE-PUBLIC-KEY to
`epg-tests-find-usable-gpg-configuration' and call it before
`epg-make-context' so that the latter uses the resulting cached
config.
(epg-tests-find-usable-gpg-configuration): Only allow gpg 2.0 for
symmetric operations.  Clear `epg--configurations' and don't pass
NO-CACHE to `epg-find-configuration'.
(epg-tests--config-program-alist): Use copy-tree to avoid modifying
the epg-config--program-alist cons values.
* test/lisp/emacs-lisp/package-tests.el (package-test-signed): Allow
running with gpg 2.0.