Expand lava-test-case docs
Expand lava-test-case docs

Make it clear that lava-test-case can be called
from inside custom scripts and LAVA will pick up
the generated results. Provide an example.

Change-Id: I3e1945bca8b5786dee4172bc9777f5285fa1d8e5

Linaro/lava-server,OSSystems/lava-server,Linaro/lava-server,OSSystems/lava-server,OSSystems/lava-server,Linaro/lava-server,Linaro/lava-server