Add experimental API test app back to unit tests.
Add experimental API test app back to unit tests.

Also restores fail-under=$MINCOVERAGE

integration/experimental_api_test_app.py is used in the integration
tests, but previously it was being run as a unit test as well.

Moving it into integration/ removed it from the unit tests and
reduced coverage slightly.

The test appears to be mostly a no-op at this point, but invoking
it from run_unit_tests.sh should reduce the coverage drop (though
there still seems to be a drop on test_vlan.py?)

REANNZ/faucet,faucetsdn/faucet,mwutzke/faucet,faucetsdn/faucet,shivarammysore/faucet,REANNZ/faucet,trungdtbk/faucet,trentindav/faucet,anarkiwi/faucet,mwutzke/faucet,shivarammysore/faucet,trentindav/faucet,trungdtbk/faucet,anarkiwi/faucet,gizmoguy/faucet,gizmoguy/faucet