test_minitest_unit.rb: invoke by array form

* test/minitest/test_minitest_unit.rb (test_capture_subprocess_io):
  invoke by array form.  quoting is not portable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46116 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/minitest/test_minitest_unit.rb

@@ -1343,8 +1343,8 @@ class TestMiniTestUnitTestCase < MiniTest::Unit::TestCase
 
     non_verbose do
       out, err = capture_subprocess_io do
-        system("echo 'hi'")
-        system("echo 'bye!' 1>&2")
+        system("echo", "hi")
+        system("echo", "bye!", out: :err)
       end
 
       assert_equal "hi\n", out