Include snapshot list unit test

This commit introduces a new set of unit tests for the snapshot list
command.

test/unit/plugins/commands/snapshot/command/list_test.rb

@@ -0,0 +1,83 @@
+require File.expand_path("../../../../../base", __FILE__)
+
+require Vagrant.source_root.join("plugins/commands/snapshot/command/list")
+
+describe VagrantPlugins::CommandSnapshot::Command::List do
+  include_context "unit"
+
+  let(:iso_env) do
+    # We have to create a Vagrantfile so there is a root path
+    env = isolated_environment
+    env.vagrantfile("")
+    env.create_vagrant_env
+  end
+
+  let(:guest)   { double("guest") }
+  let(:host)    { double("host") }
+  let(:machine) { iso_env.machine(iso_env.machine_names[0], :dummy) }
+
+  let(:argv) { [] }
+
+  subject { described_class.new(argv, iso_env) }
+
+  before do
+    allow(machine.provider).to receive(:capability?).with(:snapshot_list).
+      and_return(true)
+
+    allow(machine.provider).to receive(:capability).with(:snapshot_list).
+      and_return([])
+
+    allow(subject).to receive(:with_target_vms) { |&block| block.call machine }
+  end
+
+  describe "execute" do
+    context "with an unsupported provider" do
+      let(:argv)     { ["foo"] }
+
+      before do
+        allow(machine.provider).to receive(:capability?).with(:snapshot_list).
+          and_return(false)
+      end
+
+      it "raises an exception" do
+        machine.id = "foo"
+        expect { subject.execute }.
+          to raise_error(Vagrant::Errors::SnapshotNotSupported)
+      end
+    end
+
+    context "with a vm given" do
+      let(:argv)     { ["foo"] }
+
+      it "prints a message if the vm does not exist" do
+        machine.id = nil
+
+        expect(iso_env.ui).to receive(:info).with { |message, _|
+          expect(message).to include("VM not created")
+        }
+        expect(machine).to_not receive(:action)
+        expect(subject.execute).to eq(0)
+      end
+
+      it "prints a message if no snapshots have been taken" do
+        machine.id = "foo"
+
+        expect(iso_env.ui).to receive(:output)
+          .with(/No snapshots have been taken yet!/, anything)
+        expect(subject.execute).to eq(0)
+      end
+
+      it "prints a list of snapshots" do
+        machine.id = "foo"
+
+        allow(machine.provider).to receive(:capability).with(:snapshot_list).
+          and_return(["foo", "bar", "baz"])
+
+        expect(iso_env.ui).to receive(:output).with(/foo/, anything)
+        expect(iso_env.ui).to receive(:output).with(/bar/, anything)
+        expect(iso_env.ui).to receive(:output).with(/baz/, anything)
+        expect(subject.execute).to eq(0)
+      end
+    end
+  end
+end