Fix compatibility with Minitest 5.19+ (#289)

The `MiniTest` was renamed to `Minitest`:

minitest/minitest@9a57c52

And the `MiniTest` constant is now loaded just when `MT_COMPAT` environment variable is set:

minitest/minitest@a2c6c18

lib/fog/test_helpers/minitest/assertions.rb

@@ -1,6 +1,6 @@
 require "fog/schema/data_validator"
 
-module MiniTest::Assertions
+module Minitest::Assertions
   # Compares a hash's structure against a reference schema hash and returns true
   # when they match. Fog::Schema::Datavalidator is used for the validation.
   def assert_match_schema(actual, schema, message = nil, options = {})

lib/fog/test_helpers/minitest/expectations.rb

@@ -1,3 +1,3 @@
-module MiniTest::Expectations
+module Minitest::Expectations
   infect_an_assertion :assert_match_schema, :must_match_schema, :reverse
 end

spec/service_spec.rb

@@ -172,7 +172,7 @@ class Mock; def initialize(*_args); end; end
 
   describe "when config object can configure the service itself" do
     it "ignores the global and its values" do
-      @config = MiniTest::Mock.new
+      @config = Minitest::Mock.new
       def @config.config_service?;  true; end
       def @config.nil?; false; end
       def @config.==(other); object_id == other.object_id; end