Remove pointless warnings when running tests

This removes warnings that were complicating the test output. All of the
warnings were about other gems, except for one minor deprecation.

Rakefile

@@ -11,7 +11,7 @@ end
 Rake::TestTask.new("test") do |t|
   t.libs << "test"
   t.test_files = FileList["test/**/*_test.rb"]
-  t.verbose = true
+  t.warning = false
 end
 task default: :test
 

test/test_helper.rb

@@ -14,7 +14,7 @@
 require 'minitest/around'
 require 'rack/utils'
 require 'rack/test'
-require 'mocha/mini_test'
+require 'mocha/minitest'
 require 'timecop'
 require 'gds-api-adapters'
 require 'govuk-content-schema-test-helpers'