Use Minitest for Rails testing

Use Minitest for all tests. Do not add RSpec to the project.

Minitest is:
- Rails default, zero configuration
- Faster boot time
- Simpler syntax, less magic
- Already integrated with fixtures

```ruby
class UserTest < ActiveSupport::TestCase
test "validates email presence" do
user = User.new(name: "Test")
assert_not user.valid?
assert_includes user.errors[:email], "can't be blank"
end
end
```

Use fixtures for test data - they're fast and simple.