Adds simple Budget specs

spec/models/budget_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+describe Budget do
+  it "validates the phase" do
+    budget = create(:budget)
+    Budget::VALID_PHASES.each do |phase|
+      budget.phase = phase
+      expect(budget).to be_valid
+    end
+
+    budget.phase = 'inexisting'
+    expect(budget).to_not be_valid
+  end
+end
+