Add a new function to delete data after each test case

[cheonglam]

Like

fixtures.Delete(), which will clean all the tables

Now, koad a fixtures with null yaml file will lead to "testfixtures: fixture is not a slice or map" error

I want a delegate method to clear the database

[take]

This might be possible by using the code below:

https://github.com/go-testfixtures/testfixtures/blob/0276a8f9f40ca6c5466287655472b2349c79182d/testfixtures.go#L360-L370