Stub bundle install in install spec

spec/unit/hanami/cli/commands/app/install_spec.rb

@@ -1,15 +1,22 @@
 # frozen_string_literal: true
 
 RSpec.describe Hanami::CLI::Commands::App::Install do
-  subject { described_class.new(out: stdout) }
-  let(:stdout) { StringIO.new }
+  subject { described_class.new(fs: fs, bundler: bundler, out: out) }
+
+  let(:fs) { Hanami::CLI::Files.new(memory: true, out: out) }
+  let(:bundler) { Hanami::CLI::Bundler.new(fs: fs) }
+  let(:out) { StringIO.new }
 
   describe "#call" do
     it "installs third-party plugins" do
+      expect(bundler).to receive(:install!)
+        .exactly(1).time
+        .and_return(true)
+
       subject.call
 
-      stdout.rewind
-      expect(stdout.read.chomp).to eq("")
+      out.rewind
+      expect(out.read.chomp).to eq("")
     end
   end
 end