[libsanitizer] Fixed incorrect handling of pre-existing threads in ThreadLister test.
Also, extended the test to check that ThreadLister::Reset() works as intended.

Patch by Sergey Matveev (earthdok@google.com)


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177060 91177308-0d34-0410-b5e6-96231b3b80d8