To run the entire test suite:

bundle exec rake test

To run a single test file:

ruby -Itest test/unit/user_test.rb

To run a single test within a file:

ruby -Itest test/unit/user_test.rb -n test_create_new_password

You may also run the individual sections of the test suite:

bundle exec rake test:units
bundle exec rake test:functionals
bundle exec rake test:integration

For more info on Test::Unit: http://guides.rubyonrails.org/testing.html