Add specs to ensure parser required check correct behaviour

spec/unit/parser/required_check_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe TTY::Option::Parser::RequiredCheck do
+  it "passes required check" do
+    param = TTY::Option::Parameter::Option.create(:foo, required: true)
+    errors = []
+    aggregator = ->(err) { errors << err }
+    required_check = described_class.new(aggregator)
+    required_check << param
+
+    required_check.delete(param)
+
+    required_check.()
+
+    expect(errors).to eq([])
+  end
+
+  it "fails required check" do
+    param = TTY::Option::Parameter::Option.create(:foo)
+    errors = []
+    aggregator = ->(err) { errors << err }
+    required_check = described_class.new(aggregator)
+    required_check << param
+
+    required_check.()
+
+    error = TTY::Option::MissingParameter.new("need to provide '--foo' option")
+
+    expect(errors).to eq([error])
+  end
+end