Workaround test trace output dir creation race condition

Since dosep.ty started invoking multiple tests in parallel, the FreeBSD
buildbot occasionally has a failure due to os.mkdir returning EEXIST.
Silently ignore that exception, but reraise any other.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@205246 91177308-0d34-0410-b5e6-96231b3b80d8