Improve ThreadedQuarantineTest heap memory measurements.

[alekseyshl]

Summary

Warm up ASAN caches in ThreadedQuarantineTest to get more predictable
incremental heap memory usage measurements.

[eugenis]

LGTM

compiler-rt/trunk/lib/asan/tests/asan_noinst_test.cc

for (size_t i = 0; i < 1000; i++) {
     __asan::asan_free(p, &stack, __asan::FROM_MALLOC);
   }
   return NULL;
 }
 
 // Check that the thread local allocators are flushed when threads are
 // destroyed.
 TEST(AddressSanitizer, ThreadedQuarantineTest) {
+  // Run the routine once to warm up ASAN internal structures to get more
+  // predictable incremental memory changes.
+  pthread_t t;
+  PTHREAD_CREATE(&t, NULL, ThreadedQuarantineTestWorker, 0);
+  PTHREAD_JOIN(t, 0);
+
   const int n_threads = 3000;
   size_t mmaped1 = __sanitizer_get_heap_size();
   for (int i = 0; i < n_threads; i++) {
     pthread_t t;
     PTHREAD_CREATE(&t, NULL, ThreadedQuarantineTestWorker, 0);
     PTHREAD_JOIN(t, 0);
     size_t mmaped2 = __sanitizer_get_heap_size();
     // Figure out why this much memory is required.
-    EXPECT_LT(mmaped2 - mmaped1, 352U * (1 << 20));
+    EXPECT_LT(mmaped2 - mmaped1, 320U * (1 << 20));
   }
 }
 
 void *ThreadedOneSizeMallocStress(void *unused) {
   (void)unused;
   BufferedStackTrace stack;
   stack.trace_buffer[0] = 0x890;
   stack.size = 1;