Use latest simplecov for better coverage results

[mrcasals]

What

When developing #5934, we found a problem: when running tests for the decidim-proposals engine, where we know code from decidim-core is executed, the code in decidim-core was not considered as covered. This reduced the % of covered code in the results. This was reported in simplecov-ruby/simplecov#885.

The Simplecov maintainers have worked on this issue and have released a PR that should fix the problem: simplecov-ruby/simplecov#894. We should try this in our repo and see if it actually fixed it.

Steps to reproduce

1. Use simplecov from master branch
2. bundle install
3. bin/rake test_app
4. CI=1 SIMPLECOV=true bin/rspec ./decidim-proposals/spec/system/proposals_spec.rb:50
5. Check the coverage/coverage.xml file. With the fix, this file should include files outside decidim-proposals

[andreslucena]

We should try this in our repo and see if it actually fixed it.

I've tried it, it looks like it works well! 🥳

 $ wc -l coverage/coverage.xml.master coverage/coverage.xml.0.18.5 
  19329 coverage/coverage.xml.master
   3039 coverage/coverage.xml.0.18.5
  22368 total

Steps:

bundle install
bin/rake test_app
CI=1 SIMPLECOV=true bin/rspec ./decidim-proposals/spec/system/proposals_spec.rb:50
mv coverage/coverage.xml coverage/coverage.xml.0.18.5
echo 'gem "simplecov", github: "colszowka/simplecov", branch: "master", group: :test' >> Gemfile 
bundle install
CI=1 SIMPLECOV=true bin/rspec ./decidim-proposals/spec/system/proposals_spec.rb:50
mv coverage/coverage.xml coverage/coverage.xml.master

[mrcasals]

@andreslucena can you send a PR with this? 😄 I think modifying just the Gemfile would be enough 😄