Fix handling of dynamic URLs in the test builder

Fix the test builder to call the URL factory function before each test.

Before this change, the URL produced by the first test was used,
which caused problems in tests that were creating new entities
before each test, e.g. `lt.describe.whenCalledByUser`.
Merge pull request #28 from clarkorz/fix/url-doesnot-reproduce

Fix handling of dynamic URLs in the test builder

Close #28
Fix #27