Specify 'master' branch in unittest-app

Slugtester is changing to allow the checking out and testing
of a specific branch from a repo. This change deliberately uses
'master'; a forthcoming change will modify the signature of
worker's build and unittest methods to select other branches.

Change-Id: Idf05a9c2faef5b65ff5ea8ac2f8cfb2845a2e02c
Merge "Specify 'master' branch in unittest-app"