Add logger as a dependency, drop ostruct for tests

[olleolleolle]

This PR avoids a Ruby warning.

There was a warning in ruby-head (3.4) that in Ruby 3.5 logger and ostruct will not be built-in gems, but regular gems.

/home/runner/work/dalli/dalli/test/helper.rb:13: warning: logger was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0. Add logger to your Gemfile or gemspec.
/home/runner/work/dalli/dalli/test/helper.rb:14: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0. Add ostruct to your Gemfile or gemspec.

This change avoids that warning.

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In addition, the ostruct dependency wasn't in actual use, just require 'ostruct' statements. It was possible to fully omit that and pass all tests.

A second commit with a linting change was added, using the autocorrect feature of RuboCop.

[petergoldstein]

@olleolleolle can you please address the Rubocop warnings? Thanks.

[olleolleolle]

@petergoldstein Thanks, I revisited the change, and was able to remove the ostruct, and pass linting.

EDIT: I guess the port-finding is a little finicky, and sometimes trips tests up. Thought of this and remembered #1000 (draft) which tried to address this.