Add sign-in system test (#302)

Exercises the capybara-lockstep system-test harness end-to-end:
sign in through the browser and land on the dashboard. Matches the
example test now generated by inertia-rails/generator (#18).

Author: skryukov

spec/system/sessions_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "Sessions", type: :system do
+  fixtures :users
+
+  it "signs in and shows the dashboard" do
+    visit sign_in_path
+
+    fill_in "Email address", with: users(:one).email
+    fill_in "Password", with: "Secret1*3*5*"
+    click_on "Log in"
+
+    expect(page).to have_current_path(dashboard_path)
+    expect(page).to have_text("Dashboard")
+  end
+end