tests: fix test coverage reporting for parallel testing.

* Fix simplecov clobbering test coverage reports when using parallel tests.
* Generate coverage reports by default (remove $SIMPLECOV flag).
* Store coverage reports in tmp/coverage/ instead of coverage/.
* Enable branch coverage.

ref: github.com /colszowka/simplecov/issues/718#issuecomment-538201587

.gitignore

@@ -7,7 +7,6 @@ config/database.yml
 config/danbooru_local_config.rb
 config/deploy/*.rb
 config/newrelic.yml
-coverage/
 node_modules/
 log/
 tmp/

.simplecov

@@ -0,0 +1,6 @@
+SimpleCov.start "rails" do
+  add_group "Libraries", ["app/logical", "lib"]
+  add_group "Presenters", "app/presenters"
+  enable_coverage :branch
+  coverage_dir "tmp/coverage"
+end

test/test_helper.rb

@@ -1,13 +1,6 @@
 ENV["RAILS_ENV"] = "test"
 
-if ENV["SIMPLECOV"]
-  require 'simplecov'
-  SimpleCov.start 'rails' do
-    add_group "Libraries", ["app/logical", "lib"]
-    add_group "Presenters", "app/presenters"
-  end
-end
-
+require 'simplecov'
 require File.expand_path('../config/environment', __dir__)
 require 'rails/test_help'
 require 'cache'
@@ -48,8 +41,14 @@ class ActiveSupport::TestCase
   mock_pool_version_service!
 
   parallelize
-  parallelize_setup do
+  parallelize_setup do |worker|
     Rails.application.load_seed
+
+    SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
+  end
+
+  parallelize_teardown do |worker|
+    SimpleCov.result
   end
 
   setup do