Fix promises-aplus testcfg.py

Fix GetSourceForTest which returned an incorrect result.
With this fix promises-aplus shows correct sources when --cat option is specified.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/258763002

Patch from Yutaka Hirano <yhirano@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21026 ce2b1a6d-e550-0410-aec6-3dcde31c8c00