Remove plugin_name_v2 and extension_manager in test_config

There are two ways to specify a core plugin and an extension manager
in the unit tests: test_config and arguments of the constructor.
Both are used and it sometimes makes it a bit difficult to debug.
This patch removes the way via test_config and makes constructor
arguments the only way to do it.

Also removes the default entries in test_config because they are
not used anywhere.

Change-Id: I491003f4246e62001e9b74743c8fbfa922088383
Closes-Bug: #1245335
Merge "Remove plugin_name_v2 and extension_manager in test_config"