Add autopkgtests (#955259)

• Make the PKCS self-test even simpler

  (I've also forwarded this version of !1 (merged) upstream)

• Install test data so that the installed-tests can pass

  Patch cherry-picked from upstream.

• Add some simple autopkgtest coverage (Closes: #955259)

• Update changelog

  (This includes a changelog entry for !2 (merged))