Fix test_file? function, ./test/**/*_test.rb should be a test file too.

`rake test ./test/**/*_test.rb` should be run the single test

```ruby
require 'test_helper'

class PostTest < ActiveSupport::TestCase
  test "the truth" do
    assert true
  end
end
```

before:

```ruby
=> rake test ./test/models/post_test.rb
Run options: --seed 7653

# Running:

........

Finished in 0.108402s, 73.7994 runs/s, 129.1489 assertions/s.

8 runs, 14 assertions, 0 failures, 0 errors, 0 skips
```

after:

```ruby
=> rake test ./test/models/post_test.rb
Run options: --seed 8353

# Running:

.

Finished in 0.006665s, 150.0375 runs/s, 150.0375 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
```