Add helper method specs from RSpec docs

spec/rspec/core/helper_methods_spec.cr

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+require "../../spec_helper"
+
+Spectator.describe "Arbitrary helper methods" do
+  context "Use a method define in the same group" do
+    describe "an example" do
+      def help
+        :available
+      end
+
+      it "has access to methods define in its group" do
+        expect(help).to eq(:available) # TODO: `be` matcher with value (no same? method).
+      end
+    end
+  end
+
+  context "Use a method defined in a parent group" do
+    describe "an example" do
+      def help
+        :available
+      end
+
+      describe "in a nested group" do
+        it "has access to methods defined in its parent group" do
+          expect(help).to eq(:available) # TODO: `be` matcher with value (no same? method).
+        end
+      end
+    end
+  end
+end

spec/rspec/core/let_spec.cr

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+require "../../spec_helper"
+
+# Examples taken from:
+# https://relishapp.com/rspec/rspec-core/v/3-8/docs/helper-methods/let-and-let
+# and modified to fit Spectator and Crystal.
+Spectator.describe "Let and let!" do
+  context "Use `let` to define memoized helper method" do
+    # Globals aren't supported, use class variables instead.
+    @@count = 0
+
+    describe "let" do
+      let(:count) { @@count += 1 }
+
+      it "memoizes thte value" do
+        expect(count).to eq(1)
+        expect(count).to eq(1)
+      end
+
+      it "is not cached across examples" do
+        expect(count).to eq(2)
+      end
+    end
+  end
+
+  context "Use `let!` to define a memoized helper method that is called in a `before` hook" do
+    # Globals aren't supported, use class variables instead.
+    @@count = 0
+
+    describe "let!" do
+      # Use class variable here.
+      @@invocation_order = [] of Symbol
+
+      let!(:count) do
+        @@invocation_order << :let!
+        @@count += 1
+      end
+
+      it "calls the helper method in a before hook" do
+        @@invocation_order << :example
+        expect(@@invocation_order).to eq([:let!, :example])
+        expect(count).to eq(1)
+      end
+    end
+  end
+end