Add RSpec change matcher spec

spec/rspec/expectations/change_matcher_spec.cr

@@ -0,0 +1,49 @@
+require "../../spec_helper"
+
+# Examples taken from:
+# https://relishapp.com/rspec/rspec-expectations/v/3-8/docs/built-in-matchers/change-matcher
+# and modified to fit Spectator and Crystal.
+Spectator.describe "`change` matcher" do
+  # Modified this example type to work in Crystal.
+  module Counter
+    extend self
+
+    @@count = 0
+
+    def increment
+      @@count += 1
+    end
+
+    def count
+      @@count
+    end
+  end
+
+  context "expect change" do
+    describe "Counter#increment" do # TODO: Allow multiple arguments to context/describe.
+      it "should increment the count" do
+        expect { Counter.increment }.to change { Counter.count }.from(0).to(1)
+      end
+
+      # deliberate failure
+      # TODO: Add support for expected failures.
+      xit "should increment the count by 2" do
+        expect { Counter.increment }.to change { Counter.count }.by(2)
+      end
+    end
+  end
+
+  context "expect no change" do
+    describe "Counter#increment" do # TODO: Allow multiple arguments to context/describe.
+    # deliberate failures
+    # TODO: Add support for expected failures.
+      xit "should not increment the count by 1 (using not_to)" do
+        expect { Counter.increment }.not_to change { Counter.count }
+      end
+
+      xit "should not increment the count by 1 (using to_not)" do
+        expect { Counter.increment }.to_not change { Counter.count }
+      end
+    end
+  end
+end