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

[pcc]

Summary

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.

[merge_guards_bot]

Build result: pass - 60329 tests passed, 0 failed and 732 were skipped.

Log files: console-log.txt, CMakeCache.txt

compiler-rt/lib/scudo/standalone/tsd_shared.h

template <class Allocator, u32 MaxTSDCount> struct TSDRegistrySharedT {
   void init(Allocator *Instance) {
     memset(this, 0, sizeof(*this));
     initLinkerInitialized(Instance);
   }
 
   void unmapTestOnly() {
     unmap(reinterpret_cast<void *>(TSDs),
           sizeof(TSD<Allocator>) * NumberOfTSDs);
+    setCurrentTSD(nullptr);
   }
 
   ALWAYS_INLINE void initThreadMaybe(Allocator *Instance,
                                      UNUSED bool MinimalInit) {
     if (LIKELY(getCurrentTSD()))
       return;
     initThread(Instance);
   }