use Test::File::ShareDir in tests

WebService::NFSN called `dist_file` to discover the location of nfsn-ca.crt, but if the module was not already installed, `dist_file` would die.

This pull request updates the test suite to use Test::File::ShareDir so we use the nfsn-ca.crt in the local sibling directory.

With this pull request, "`prove -l t`" succeeds even if WebService::NFSN is not installed.

If NearlyFreeSpeech.net ends up using a standard CA-signed certificate for api.nearlyfreespeech.net, then this may be moot :-)