dotest.py: Fail if we detect multiple tests with the same name

Summary:
Log files produced by dotest have names derived from the test name, and this produces errors in
case we have multiple tests with the same name. Additionally, it's good if the test name explains
what the test is testing (which it clearly doesn't do well if there are multiple tests with
identical names). This commit makes the presence of such tests a hard error.

Reviewers: tberghammer, zturner

Subscribers: iancottrell, lldb-commits

Differential Revision: http://reviews.llvm.org/D13588

llvm-svn: 249828