Add an exception for UBSAN vptr on boost::unit_test::decorator::timeout

Summary: This is a false positive.

Test Plan:

cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug \
  -DENABLE_SANITIZERS=undefined \
  -DCMAKE_C_COMPILER=clang-8 -DCMAKE_CXX_COMPILER=clang++-8
ninja check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6784