tor-guardmgr filtering_basics test failure - Job Failed #138380

Job #138380 failed for 573d1236:

This was a test of main, after a merge. Nothing seems to have changed in the code that could possibly cause this. I conclude that this test is flaky. There is an RNG involved.

I think the causes are three problems:

1. There seems to be some underlying low-probability bug in the guard selection algorithm. (I ran the test by hand a dozen or so times and it passed each time.)

2. The test is not sufficiently comprehensive.

3. #486 (closed)