test_process.rb: fix test

* test/ruby/test_process.rb (assert_fail_too_long_path): get rid of
  syntax error on sh, increase command line size until it exceeds the
  limit.  [Bug #7904]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39417 b2dd03c8-39d4-4d8f-98ff-823fe69b080e