Test Zeitwerk setup (#13)

By eager loading in a test example we can see the loader is setup
correctly.

This has been adapted to RSpec from Zeitwerk's README about testing
compliance [1].

[1] - https://github.com/fxn/zeitwerk#testing-compliance

Author: waiting-for-dev

spec/unit/operation_spec.rb

@@ -5,6 +5,12 @@
 RSpec.describe Dry::Operation do
   include Dry::Monads[:result]
 
+  it "configures loader" do
+    expect do
+      described_class.loader.eager_load(force: true)
+    end.not_to raise_error
+  end
+
   describe "#steps" do
     it "wraps block's return value in a Success" do
       klass = Class.new(described_class) do