Fix TestUbsanBasic

Summary:
Potentially due to the recent testuite refactorings, this test now reports
a full absolute path but expect just the filename. For some reason this
test is skipped on GreenDragon so we've never seen the issue.

Reviewers: vsk

Subscribers: kubamracek, lldb-commits

Differential Revision: https://reviews.llvm.org/D43577

llvm-svn: 325859