How to set ASAN/UBSAN reporting output

I would like to run my unit test suite with -fsanitize=address,undefined and have all sanitizer errors be written to a report.txt file. By default all sanitizer errors get written to stdout, however the software also writes info to stdout so this makes it difficult to detect errors. I tried:

export ASAN_OPTIONS="log_path=asan.log"
./mytests

And I also tried calling the C API before running tests:

#include <sanitizer/asan_interface.h>

__sanitizer_set_report_path("/tmp/asan.log")

However neither seems to work, all errors just get written to stdout. I am using Debian testing:

root@94e239ad460a:~# gcc --version
gcc (Debian 6.1.1-11) 6.1.1 20160802
Copyright (C) 2016 Free Software Foundation, Inc.

Is there any alternative method that I can save the sanitizer error from my unit tests somewhere?