cpp,tests: Verify that requested keylist mode is used

* lang/cpp/tests/run-getkey.cpp, lang/cpp/tests/run-keylist.cpp (main):
Check used keylist mode.

Since Context::setKeyListMode() does not return the error returned by
gpgme, we need to verify explicitly that the keylist mode was set
successfully.

• GnuPG-bug-id: T5951