tests/unit: Initialize core plugin in TestL3GwModeMixin

tests/unit: Initialize core plugin in TestL3GwModeMixin

TestL3GwModeMixin can fail randomly because it doesn't initialize
core_plugin and can be run random core plugin depending on execution
order of tests. It also fails with core plugin uninitialized when it
is run without other tests.

This patch refactors the setup code of core plugin and apply it to the
related tests.
This patch reveled the same bug of test_metaplugin.py which is also
fixed by this patch.

Closes-bug: #1273259
Change-Id: I3c1d4d8b3d69262b89c7747daa8267bf2c8e7f6b