Add `Installer` spec.

Library/Homebrew/test/cask/artifact/installer_spec.rb

+describe Hbc::Artifact::Installer, :cask do
+  let(:staged_path) { mktmpdir }
+  let(:cask) { instance_double("Cask", staged_path: staged_path, config: nil) }
+  subject(:installer) { described_class.new(cask, **args) }
+  let(:command) { Hbc::SystemCommand }
+
+  let(:args) { {} }
+
+  describe "#install_phase" do
+    context "when given a manual installer" do
+      let(:args) { { manual: "installer" } }
+
+      it "shows a message prompting to run the installer manually" do
+        expect {
+          installer.install_phase(command: command)
+        }.to output(%r{run the installer at\s+'#{staged_path}/installer'}).to_stdout
+      end
+    end
+
+    context "when given a script installer" do
+      let(:executable) { staged_path/"executable" }
+      let(:args) { { script: { executable: "executable" } } }
+
+      before(:each) do
+        FileUtils.touch executable
+      end
+
+      it "looks for the executable in HOMEBREW_PREFIX" do
+        expect(command).to receive(:run!).with(
+          executable,
+          a_hash_including(
+            env: { "PATH" => PATH.new("#{HOMEBREW_PREFIX}/bin", "#{HOMEBREW_PREFIX}/sbin", ENV["PATH"]) },
+          ),
+        )
+
+        installer.install_phase(command: command)
+      end
+    end
+  end
+end