Avoid testable_flash method be treated as a test case

The testable_flash method is being treated as a test case when using Test::Unit.
Just renamed it and added a test using Test::Unit.
If run the test without rename the method, a message like this below will appear, even having only one test case on the file.

```
# Running tests:

.E

Finished tests in 0.006769s, 295.4507 tests/s, 147.7254 assertions/s.

  1) Error:
testable_flash(ControllerTest):
ArgumentError: wrong number of arguments (0 for 1)
    lib/cacheable_flash/test_helpers.rb:13:in `testable_flash'
```