Fix SimpleCov config for guard MiniTest

The code coverage analysis tool did not correctly report test
coverage results if executed via guard-minitest.

Externalizing the config into `.simplecov` is considered best practice:
simplecov-ruby/simplecov#235 (comment)
The `.simplecov` config is optimized for Cloud Stove based on the Rails
defaults from: https://github.com/colszowka/simplecov/blob/master/lib/simplecov/defaults.rb

Positive side effect: Adding Spring support for MiniTest accelerates
repeated test executions through reloading: https://github.com/guard/guard-minitest#spring

The new Rake task `rake test:coverage` also triggers coverage analysis.

SimpleCov is very fast (i.e., a couple of seconds for ~10 min Rails test suite)
and thus they enable coverage analysis by default. For conditional execution, see:
https://github.com/colszowka/simplecov#running-coverage-only-on-demand

.simplecov

@@ -0,0 +1,36 @@
+# https://github.com/colszowka/simplecov#using-simplecov-for-centralized-config
+# Maybe put some conditional here not to execute the code below unless ENV['COVERAGE'] == 'true'
+SimpleCov.start do
+  # see https://github.com/colszowka/simplecov/blob/master/lib/simplecov/defaults.rb
+  load_profile 'test_frameworks'
+  coverage_dir 'coverage'
+  command_name 'MiniTest'
+  merge_timeout 3600 # 1 hour
+  track_files "{app,lib}/**/*.rb"
+
+  # Groups
+  add_group 'Controllers', 'app/controllers'
+  add_group 'Models', 'app/models'
+  # add_group 'Mailers', 'app/mailers'
+  add_group 'Helpers', 'app/helpers'
+  add_group 'Jobs', %w(app/jobs app/workers)
+  # add_group 'Libraries', 'lib'
+
+  add_group 'Provider updater', 'app/provider_updater'
+  add_group 'Long files' do |src_file|
+    src_file.lines.count > 100
+  end
+  class MaxLinesFilter < SimpleCov::Filter
+    def matches?(source_file)
+      source_file.lines.count < filter_argument
+    end
+  end
+  add_group 'Short files', MaxLinesFilter.new(5)
+
+  # Exclude these paths from analysis
+  add_filter '/config/'
+  add_filter '/db/'
+  add_filter 'lib/plugins'
+  add_filter 'vendor'
+  add_filter 'bundle'
+end

Gemfile

@@ -70,7 +70,7 @@ group :development, :test do
   gem 'webmock'
 
   gem 'minitest-reporters'
-  gem 'simplecov', :require => false
+  gem 'simplecov', require: false
 
   # Acceptance test framework for web applications: https://github.com/jnicklas/capybara
   gem 'capybara'

Guardfile

@@ -27,7 +27,7 @@ guard :bundler do
   files.each { |file| watch(helper.real_path(file)) }
 end
 
-guard :minitest do
+guard :minitest, spring: true do
   # with Minitest::Unit
   watch(%r{^test/(.*)\/?test_(.*)\.rb$})
   watch(%r{^lib/(.*/)?([^/]+)\.rb$})     { |m| "test/#{m[1]}test_#{m[2]}.rb" }

lib/tasks/simplecov.rake

@@ -0,0 +1,7 @@
+# Usage: `rake test:coverage`
+namespace :test do
+  task :coverage do
+    require 'simplecov'
+    Rake::Task['test'].execute
+  end
+end

test/test_helper.rb

@@ -1,5 +1,4 @@
 require 'simplecov'
-SimpleCov.start 'rails'
 
 ENV['RAILS_ENV'] ||= 'test'
 require File.expand_path('../../config/environment', __FILE__)