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