Fix broken test_lunch.py

The orchestrator path inside the test workspace (test/configs) is
build/make, which is different from the current path in multitree
(build/build/make). This was causing `python test_lunch.py` to fail

Rather than restructing the test workspace to build/build/make now (and
build/ later when the orchestrator is in its own project), create a
Context object which can be mocked.

Test: python test_lunch.py
Change-Id: Ibd89c3ff0204ebad320c90a05bab4c0f8c86b3a0