Spec for problem 15

spec/problem-15.rb

@@ -0,0 +1,12 @@
+require_relative 'spec_helper'
+require_relative '../solutions/solution-15'
+
+describe 'Lattice paths' do
+  it 'works for the given example' do
+    expect(Solution15.new(2).solution).to eq(6)
+  end
+
+  it 'works for the known answer' do
+    expect(Solution15.new(20).solution).to eq(137846528820)
+  end
+end