Add spec for 'rake db:dev_seed' task

spec/lib/tasks/dev_seed_spec.rb

@@ -0,0 +1,18 @@
+require 'rails_helper'
+require 'rake'
+
+describe 'rake db:dev_seed' do
+  before do
+    Rake.application.rake_require('tasks/db')
+    Rake::Task.define_task(:environment)
+  end
+
+  let :run_rake_task do
+    Rake::Task['db:dev_seed'].reenable
+    Rake.application.invoke_task('db:dev_seed')
+  end
+
+  it 'seeds the database without errors' do
+    expect { run_rake_task }.to_not raise_error
+  end
+end