Intermittent test failures in dir/dirserv_read_measured_bandwidths

I observed this failure once out of about 10 tests when testing #29541 (moved) on commit:

commit 5614960e94 (HEAD, tor-github/pr/723/merge)
Merge: 69238ca2da 065e7da8e6

Here is the test error:

dir/dirserv_read_measured_bandwidths: [forking] 
  FAIL ../src/test/test_dir.c:1802: assert(0 OP_EQ dirserv_read_measured_bandwidths(fname, NULL, bw_file_headers, NULL)): 0 vs -1
  [dirserv_read_measured_bandwidths FAILED]

It looks like this test was last modified in #26698 (moved) in master.

Is it an obvious fix? If not, let's see if it happens again.