Implement rake task to combine results

lib/tasks/simplecov.rake

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+namespace :simplecov do
+  desc "Collates all result sets produced during parallel test runs"
+  task :collate_results, # rubocop:disable Rails/RakeEnvironment doesn't need the full env
+       [:path_to_results, :coverage_dir] do |_t, args|
+    require "simplecov"
+
+    path_to_results = args[:path_to_results].presence || "tmp/simple-cov"
+    coverage_dir = args[:coverage_dir].presence || "coverage"
+
+    SimpleCov.collate Dir[File.join(path_to_results, "**", ".resultset.json")], "rails" do
+      formatter SimpleCov::Formatter::HTMLFormatter
+
+      coverage_dir coverage_dir
+    end
+  end
+end