guests/atomic: Add tests for docker capability

This commit adds missing test coverage for the Docker capability on
atomic guests.

test/unit/plugins/guests/atomic/cap/docker_test.rb

@@ -0,0 +1,28 @@
+require_relative "../../../../base"
+
+describe "VagrantPlugins::GuestAtomic::Cap::ChangeHostName" do
+  let(:described_class) do
+    VagrantPlugins::GuestAtomic::Plugin
+      .components
+      .guest_capabilities[:atomic]
+      .get(:docker_daemon_running)
+  end
+
+  let(:machine) { double("machine") }
+  let(:comm) { VagrantTests::DummyCommunicator::Communicator.new(machine) }
+
+  before do
+    allow(machine).to receive(:communicate).and_return(comm)
+  end
+
+  after do
+    comm.verify_expectations!
+  end
+
+  describe ".docker_daemon_running" do
+    it "checks /run/docker/sock" do
+      described_class.docker_daemon_running(machine)
+      expect(comm.received_commands[0]).to eq("test -S /run/docker.sock")
+    end
+  end
+end