scudo: Call setCurrentTSD(nullptr) when bringing down the TSD registry in tests.

Otherwise, we will hit a use-after-free when testing multiple instances of
the same allocator on the same thread. This only recently became a problem
with D70552 which caused us to run both ScudoCombinedTest.BasicCombined and
ScudoCombinedTest.ReleaseToOS on the unit tests' main thread.

Differential Revision: https://reviews.llvm.org/D70760