x86-generic-asan time out on vmtests, e.g.

https://build.chromium.org/p/chromiumos/builders/x86-generic-asan/builds/24223

There are no obvious failures, it seems we're just running too slowly and get killed by the 1 hour test timeout:

23:29:01: ERROR: Traceback (most recent call last):
  File "/b/cbuild/repository/chromite/cbuildbot/stages/generic_stages.py", line 628, in Run
    self.PerformStage()
  File "/b/cbuild/repository/chromite/cbuildbot/stages/test_stages.py", line 213, in PerformStage
    self._RunTest(test_type, per_test_results_dir)
  File "/b/cbuild/repository/chromite/cbuildbot/stages/test_stages.py", line 199, in _RunTest
    ssh_private_key=ssh_private_key)
  File "/b/cbuild/repository/chromite/cbuildbot/commands.py", line 777, in RunTestSuite
    kill_timeout=10 * 60)
  File "/b/cbuild/repository/chromite/lib/cros_build_lib.py", line 593, in RunCommand
    (cmd_result.output, cmd_result.error) = proc.communicate(input)
  File "/usr/lib/python2.7/subprocess.py", line 796, in communicate
    self.wait()
  File "/usr/lib/python2.7/subprocess.py", line 1376, in wait
    pid, sts = _eintr_retry_call(os.waitpid, self.pid, 0)
  File "/usr/lib/python2.7/subprocess.py", line 476, in _eintr_retry_call
    return func(*args)
  File "/b/cbuild/repository/chromite/lib/timeout_util.py", line 62, in kill_us
    raise TimeoutError(error_message % {'time': max_run_time})
TimeoutError: Timeout occurred- waited 3600 seconds. Reached VMTestStage test run timeout.

We'll need to investigate how to raise the timeout.