memory_status.rb: find proper ps

* test/ruby/memory_status.rb (Memory::PSCMD): use ps command which
  outputs expected result.  [ruby-dev:48370] [Bug #10010]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46755 b2dd03c8-39d4-4d8f-98ff-823fe69b080e