[wrt]update testsprefix for manifest and packertool

[Detailed commit message (if have)]
Impacted TCs num(approved): New 0, update 0, deleted 0
Unit test platform: [tizen]
Unit test result summary:
tcs run block,BUGID:https://crosswalk-project.org/jira/browse/XWALK-1590
no tcs change, update README and execute path, pack and run the python script ok.
Merge pull request #871 from linshen/shenlin/wrt/optissue

[wrt]update testsprefix for manifest and packertool