let blocks defined in Minitest::Spec files will be unintentionally run as tests

I suspect that this owes to the way you define tests in Minitest::Test and Test::Unit before it, but if you define a `let` in Minitest::Spec with a name that begins with `test_`, it will be inadvertently executed as one of the tests.  This can have unintended consequences.  Maybe `let` can just raise an error if called with a name that begins with `test_`?

```ruby
require 'minitest/autorun'

describe "lets that can be run as tests" do
  let(:test_that_this_is_run_because_of_the_name) do
    puts "I ran"
  end
end
```