fire up the console in the test env:
RAILS_ENV=test script/console
then require a Ruby test runner:
require 'test/unit/ui/console/testrunner'
(should be loaded but doesn't hurt)
then manually load your test file, like:
load 'tests/some_controller_test.rb
and run it by passing the test class to the test runner:
Test::Unit::UI::Console::TestRunner.run SomeControllerTest
nb: you have to reload the test file every time you make a change