Refactor testing code

[masomel]

There seems to be a lot of repeated code used for testing (e.g. constructing a tree with 3 entries). It might be worth moving these repeated code segments into appropriate functions into a separate module?

cc @c633 @arlolra

[liamsi]

As I did some moving around of test code, I could take care of this. Does it make sense to start this soon? I would create a (possibly internal) testutils package where I would the methods that are used in different packages.

[vqhuy]

Sounds good to me!

[vqhuy]

• Tests in auditlog_test.go should be rewritten and integrated with the directory tests, after our APIs are complete.

• Rewrite removed consistencychecks_test.go and directory_test.go's tests.

  • Registration tests (after [WIP] Registration should happen in epoch #189 gets merged).
  • KeyLookup tests.

[masomel]

• Refactor more common testing functionality, to create a simpler testing API, in which developers can directly pass test cases (see Refactor testings code #191 (comment))