Follow-up from "Draft: Resolve "95-pg-cb-v4-1" refactor unit test classes

There is a lot of common code between unit tests and classes in pgsql_cb_dhcp4_unittest.cc and mysql_cb_dhcp4_unitttest.cc. This should be refactored.

The following discussion from !1539 (merged) should be addressed:

@marcin started a discussion: (+2 comments)

Did you consider some form of unification of this class with the MySQL specific test class? Just asking. I realize it may complicate things.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information