Tweak assert_equal argument order in tests

The argument order for `assert_equal` is expected, actual:
https://github.com/minitest/minitest/blob/v5.25.5/lib/minitest/assertions.rb#L214

This PR corrects the arguments that were reversed to the expected order.
It also enables `Minitest/LiteralAsActualArgument`, which can detect and
autocorrect some of these cases:
https://docs.rubocop.org/rubocop-minitest/0.38/cops_minitest.html#minitestliteralasactualargument

.rubocop.yml

@@ -7,3 +7,6 @@ plugins:
 
 Gemspec/DevelopmentDependencies:
   Enabled: true
+
+Minitest/LiteralAsActualArgument:
+  Enabled: true