automake: Use parallel test harness to prevent a parallel make issue

automake: Use parallel test harness to prevent a parallel make issue

The http, post and json test are using the same TCP port. Because of
that we need to specify an execution order or we will get errors like

  INFO  WEBrick 1.3.1
  INFO  ruby 2.0.0 (2014-11-13) [x86_64-linux]
  WARN  TCPServer Error: Address already in use - bind(2)
  WARN  TCPServer Error: Address already in use - bind(2)
  /usr/lib64/ruby/2.0.0/webrick/utils.rb:85:in `initialize': Address already in use - bind(2) (Errno::EADDRINUSE)
          from /usr/lib64/ruby/2.0.0/webrick/utils.rb:85:in `new'
          from /usr/lib64/ruby/2.0.0/webrick/utils.rb:85:in `block in create_listeners'
          from /usr/lib64/ruby/2.0.0/webrick/utils.rb:82:in `each'
          from /usr/lib64/ruby/2.0.0/webrick/utils.rb:82:in `create_listeners'
          from /usr/lib64/ruby/2.0.0/webrick/server.rb:132:in `listen'
          from /usr/lib64/ruby/2.0.0/webrick/server.rb:113:in `initialize'
          from /usr/lib64/ruby/2.0.0/webrick/httpserver.rb:45:in `initialize'
          from ./unittest_http.rb:184:in `new'
          from ./unittest_http.rb:184:in `<main>'

when running the tests in parallel (`make -j5 check`).