Add test coverage for EmberCli::App#test

Always pass `--environment test` to `ember test` command from Rails.

Resolve testing issue by adding test coverage.

Required [rondale-sc/ember-cli-rails-addon#20] to correct fingerprinting
issue.

[#20]: rondale-sc/ember-cli-rails-addon#20

lib/ember-cli/command.rb

@@ -6,7 +6,7 @@ def initialize(paths:, options: {})
     end
 
     def test
-      "#{paths.ember} test"
+      "#{paths.ember} test --environment test"
     end
 
     def build(watch: false)

spec/lib/ember-cli/app_spec.rb

@@ -0,0 +1,9 @@
+require "ember-cli-rails"
+
+describe "User tests Ember app" do
+  it "exits with exit status of 0" do
+    passed = EmberCli["my-app"].test
+
+    expect(passed).to be true
+  end
+end