test_l2population: Use a fake mech driver instead of ofagent

test_l2population: Use a fake mech driver instead of ofagent

As ofagent mech driver is subject to be moved out of neutron tree
for vendor code decomposition, use a fake mech driver for UTs
instead.

An alternative to remove the relevant test cases was considered.
But it's still nice to have test cases involving multiple drivers.

Anyway using fake drivers rather than real ones for UTs like this
is a good thing.

Partial-Bug: #1412653
Related: blueprint core-vendor-decomposition
Change-Id: I4b0d344da9ccbda79482f64535396fcc1a495d0f