Add tests for rails runner

* Refs rails#42007

Author: iridakos

railties/test/commands/runner_test.rb

@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "rails/command"
+require "rails/commands/runner/runner_command"
+
+class Rails::RunnerTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::Isolation
+
+  setup :build_app
+  teardown :teardown_app
+
+  def test_rails_runner_with_stdin
+    command_output = `echo "puts 'Hello world'" | #{app_path}/bin/rails runner -`
+
+    assert_equal <<~OUTPUT, command_output
+      Hello world
+    OUTPUT
+  end
+
+  def test_rails_runner_with_file
+    # We intentionally define a file with a name that matches the one of the
+    # script that we want to run to ensure that runner executes the latter one.
+    app_file "lib/foo.rb", "# Lib file"
+
+    app_file "foo.rb", <<-RUBY
+    puts "Hello world"
+    RUBY
+
+    assert_equal <<~OUTPUT, run_runner_command("foo.rb")
+      Hello world
+    OUTPUT
+  end
+
+  def test_rails_runner_with_ruby_code
+    assert_equal <<~OUTPUT, run_runner_command('puts "Hello world"')
+      Hello world
+    OUTPUT
+  end
+
+  def test_rails_runner_with_syntax_error_in_ruby_code
+    command_output = run_runner_command("This is not ruby code", allow_failure: true)
+
+    assert_match(/Please specify a valid ruby command/, command_output)
+    assert_equal 1, $?.exitstatus
+  end
+
+  def test_rails_runner_with_name_error_in_ruby_code
+    assert_raise(NameError) { IDoNotExist }
+
+    command_output = run_runner_command("IDoNotExist.new", allow_failure: true)
+
+    assert_match(/Please specify a valid ruby command/, command_output)
+    assert_equal 1, $?.exitstatus
+  end
+
+  private
+    def run_runner_command(argument, allow_failure: false)
+      rails "runner", argument, allow_failure: allow_failure
+    end
+end