Add tests and CI

[willnet]

Deploying with Capistrano and systemd can be difficult to verify manually, so I automated the verification using tests and CI. I used a Debian Docker image to create a container running systemd, and deployed a small Sinatra application to confirm it works.

With mostly default settings, I confirmed that the four tasks puma:install, deploy, puma:stop, and puma:start work as expected. It would be good to also verify other tasks and the nginx configuration, but for now, this provides the minimum necessary verification.

Running the tests revealed two issues, which I was able to fix. For details, please refer to the descriptions of each commit.

All tests from Ruby 2.5 to 3.3 have passed, as shown in the following link:

Fix ci on Ruby 2.5 · willnet/capistrano-puma@9a83087

Ideally, this PR should be merged and version 6.0.0 released. However, if there is no response, I will consider releasing a forked gem.