Fix require in tests in Ruby 1.9

[ai]

Ruby 1.9 doesn’t has . in $LOAD_PATH, so we need another way to require test helper to run tests.

[mislav]

Relative requires are hard to write and maintain. The common solution to this problem is to have the test runner (in this case, the rake task) put test/ onto RUBYLIB. This is what RSpec does with spec/ directory by default.

Rake::TestTask.new do |t|
  t.libs << 'test'
  t.verbose = true
  t.warning = true
end