Implement code coverage using SimpleCov

[davispuh]

This PR implements code coverage using SimpleCov

[Copilot]

Pull request overview

This PR introduces SimpleCov-based code coverage instrumentation for both the Shindo test suite (tests/) and the Minitest suite (minitests/), with branch coverage configuration via a .simplecov profile.

Changes:

• Start SimpleCov in both tests/helper.rb and minitests/test_helper.rb, gated by ENV['COVERAGE'].
• Default ENV['COVERAGE'] to "true" in the Rakefile so rake runs collect coverage unless explicitly disabled.
• Add SimpleCov configuration (.simplecov) and declare SimpleCov as a development dependency in the gemspec.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/helper.rb	Starts SimpleCov before loading the library in the Shindo test helper.
Rakefile	Sets default COVERAGE env var to enable coverage during rake-driven test runs.
minitests/test_helper.rb	Starts SimpleCov before loading the library in the Minitest helper.
fog-libvirt.gemspec	Adds SimpleCov as a development dependency with a minimum version.
.simplecov	Defines coverage configuration (filters + branch coverage settings).

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[ekohl]

Thanks!