rebaseline_server: extract common unittest code, and rename tests/ -> testdata/

NOTRY=True
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/170803005

git-svn-id: http://skia.googlecode.com/svn/trunk@13499 2bbb7eff-a529-9590-31e7-b0007b416f81
57 files changed