Support sanitizers in runtime tests (#4622)

kotlin-native/runtime/src/main/cpp/AllocTest.cpp

@@ -112,7 +112,9 @@ TEST_F(KonanAllocatorAwareTest, PlacementAllocated) {
 
 TEST_F(KonanAllocatorAwareTest, PlacementConstructedArray) {
     constexpr size_t kCount = 5;
-    std::array<uint8_t, sizeof(A) * kCount> buffer;
+    // TODO: Consider removing support for placement new[] altogether, since there's no
+    //       portable way to know needed storage size ahead of time.
+    alignas(A) std::array<uint8_t, sizeof(A) * kCount + sizeof(size_t)> buffer;
     A* as = new (buffer.data()) A[kCount];
 
     std::vector<int> actual;

kotlin-native/runtime/src/main/cpp/TestSupport.hpp

@@ -8,6 +8,9 @@ namespace kotlin {
 #if KONAN_WINDOWS
 // TODO: Figure out why creating many threads on windows is so slow.
 constexpr int kDefaultThreadCount = 10;
+#elif __has_feature(thread_sanitizer)
+// TSAN has a huge overhead.
+constexpr int kDefaultThreadCount = 10;
 #else
 constexpr int kDefaultThreadCount = 100;
 #endif