Better configure code coverage

Coverage is not working on CodeClimate or locally, at least in part
because it is incorrectly configured. Trying to fix this with the
following changes:

* require and start coverage before loading the real code
* filter out the test files from the coverage

test/unit/test_helper.rb

@@ -3,18 +3,22 @@
 require 'rubygems'
 require 'minitest/autorun'
 $LOAD_PATH.unshift File.expand_path('../../lib')
-require 'gitdocs'
 require 'mocha/setup'
 require 'shell_tools'
-
 Dir.glob(File.expand_path('../../support/**/*.rb', __FILE__)).each do |filename|
   require_relative filename
 end
 
-Gitdocs::Initializer.root_dirname = '/tmp/gitdocs'
-Gitdocs::Initializer.database     = ':memory:'
-
+# Setup code coverage, first ###################################################
 require 'codeclimate-test-reporter'
+SimpleCov.add_filter 'test'
+SimpleCov.start
 CodeClimate::TestReporter.start
 
+# Load and configure the real code #############################################
+require 'gitdocs'
+
+Gitdocs::Initializer.root_dirname = '/tmp/gitdocs'
+Gitdocs::Initializer.database     = ':memory:'
+
 GitFactory.working_directory = File.expand_path('../../../tmp/unit', __FILE__)