sanitizer_common: fix up onprint.cpp test more

[dvyukov]

Summary

D112630 ("sanitizer_common: fix up onprint.cpp test")
added O_CREAT, but we also need O_TRUNC b/c the file
may not exist, or may exist as well.

compiler-rt/test/sanitizer_common/TestCases/onprint.cpp

 
 __attribute__((disable_sanitizer_instrumentation)) extern "C" void
 __sanitizer_on_print(const char *str) {
   write(f, str, strlen(str));
 }
 
 int main(int argc, char *argv[]) {
   assert(argc >= 2);
-  f = open(argv[1], O_CREAT | O_WRONLY, 0666);
+  f = open(argv[1], O_CREAT | O_TRUNC | O_WRONLY, 0666);
 
   // Use-after-free to trigger ASan/TSan reports.
   void *ptr = malloc(1);
   buf = ptr;
   free(ptr);
   sink = *static_cast<char *>(ptr);
   return 0;
 }