Fix validate.py and test.py traversals

The get_modified_files call needs to be run from the doc directory
since we invoce it with --relative.

I noticed that we skipped validation of files in some case
and confirmed that this is now handled properly.

Change-Id: Ieaa5bfb2f7ada6345dba8b6b4116f17dff582fe1
Closes-Bug: #1228777
Merge "Fix validate.py and test.py traversals"