Run the "false" command when given a non-existent path

This change provides a command result that is equivalent
to a failed command

lib/test/cmd.rb

@@ -55,6 +55,9 @@ def spawn
         Process.spawn(@cmd, *@argv, {out: out.w, err: err.w})
         Process.wait
         @status = $?
+      rescue Errno::ENOENT
+        @cmd, @argv = "false", []
+        retry
       end
       loop do
         io, _ = IO.select([out.r, err.r], nil, nil, 0.01)

test/cmd_test.rb

@@ -36,6 +36,10 @@ def test_ruby_exit_status_predicates
       assert_equal true, ruby("exit 0").status.success?
       assert_equal true, ruby("exit 0").success?
     end
+
+    def test_nonexistent_command
+      assert_equal false, cmd("/a/path/that/is/not/found").success?
+    end
   end
 
   ##