flaky test in testing/e2e/backfill_test.go

[joeholley]

What happened:
flaky test was removed from 1.7 -> 1.8 in testing/e2e/backfill_test.go. Diff them to see it: release-1.7...release-1.8

This is a backfill-related test, so it was community contributed. If the community could contribute a fix, that would be great!

What you expected to happen:
Test should produce the same output from the same input.

How to reproduce it (as minimally and precisely as possible):
Can't provide minimal - only uncovered this as part of running the CI/CD build, where it failed. Log of the failed build is visible in commit 78da2a6 (I dumped it in the src file as a comment).