[analyzer] SATestBuild: Don't require reference results to have logs.

The Logs directory isn't used for testing, so it's filtered out ahead of
time. However, there's then no reason to include it in version control at
all. Don't error if it's not present.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183689 91177308-0d34-0410-b5e6-96231b3b80d8