Enforce runtime dependency on Test::Synopsis

1. Inject Test::Synopsis in the `develop` dependencies of the user of the plugin to ensure the author will have it installed.
2. In the generated test, use Test::Synopsis without guard. We no longer skip if the module is not installed. Because this is a release test that will run only on the author environment, it is mandatory.

This can avoid strange bugs where the test is skipped because of a user installation issue. Instead, if the plugin is referenced in dist.ini we want to enforce that the test runs and succeed.
Here is an example of the kind of bugs that can happen:
   rjbs/Dist-Zilla#283