tsan: make positive tests more robust
Add a script that is used to deflake inherently flaky tsan tests.
It is invoked from lit tests as:
%deflake %run %t
The script runs the target program up to 10 times,
until it produces a tsan warning.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209898 91177308-0d34-0410-b5e6-96231b3b80d8