tests/key.scm: New test suite

* tests/key.scm: New test suite.
* tests/Makefile.am (SCM_TESTS): Add `key.scm'.
  (EXTRA_DIST): Add `dsakey.pub', `ecdsakey', `ecdsakey.pub',
  `rsakey.pub'.
* tests/dsakey.pub, tests/ecdsakey, tests/ecdsakey.pub,
  tests/rsakey.pub: New files
* NEWS: Update.