Add failing tests to demonstrate issues in <=>

Plus begin directly testing <=> with a separate set of tests for additional peace of mind

spec/lib/t/struct/acts_as_comparable_spec.rb

@@ -4,6 +4,74 @@
 module T
   class Struct
     describe ActsAsComparable do
+      describe '#<=>' do
+        subject(:three_way_comparison) { struct_a <=> struct_b }
+
+        let(:struct_a) do
+          SorbetStructComparable::Examples::Interest.new(
+            topic: SorbetStructComparable::Examples::Interest::Topic::Walking,
+            rating: 10
+          )
+        end
+        let(:struct_b) do
+          SorbetStructComparable::Examples::Interest.new(
+            topic: SorbetStructComparable::Examples::Interest::Topic::Walking,
+            rating: 10
+          )
+        end
+
+        context 'when structs are equal' do
+          it { is_expected.to eq 0 }
+        end
+
+        context 'when first struct is lesser than the second' do
+          let(:struct_a) do
+            SorbetStructComparable::Examples::Interest.new(
+              topic: SorbetStructComparable::Examples::Interest::Topic::Walking,
+              rating: 9
+            )
+          end
+
+          it { is_expected.to eq -1 }
+        end
+
+        context 'when first struct is greater than the second' do
+          let(:struct_a) do
+            SorbetStructComparable::Examples::Interest.new(
+              topic: SorbetStructComparable::Examples::Interest::Topic::Walking,
+              rating: 11
+            )
+          end
+
+          it { is_expected.to eq 1 }
+        end
+
+        context 'when first struct is of a different type to the second' do
+          let(:struct_a) do
+            SorbetStructComparable::Examples::Incomparable.new(
+              my_incomparable_attribute: BigDecimal(22)
+            )
+          end
+
+          it { is_expected.to be_nil }
+        end
+
+        context 'when first struct has an incomparable attribute to the second' do
+          let(:struct_a) do
+            SorbetStructComparable::Examples::Incomparable.new(
+              my_incomparable_attribute: BigDecimal(22)
+            )
+          end
+          let(:struct_b) do
+            SorbetStructComparable::Examples::Incomparable.new(
+              my_incomparable_attribute: ['Hello']
+            )
+          end
+
+          it { is_expected.to be_nil }
+        end
+      end
+
       describe '#==' do
         subject(:comparison) { person_a }
 
@@ -66,7 +134,7 @@ class Struct
           it { is_expected.to eq person_b }
         end
 
-        context 'when presented with structs that are incomparable' do
+        context 'when presented with structs that have incomparable attributes' do
           let(:person_a) do
             SorbetStructComparable::Examples::Incomparable.new(
               my_incomparable_attribute: BigDecimal(22)