commit bbf0dc0162e82770f296b2ecda77a2b5bd6f7405 "tests: Don't fail run-debuginfod-fd-prefetch-caches.sh if grep -c fails" made the test not really do anything. The debuginfod server gets to scan a non-existing directory and then nothing even tries to query anything from the server. So nothing ever gets prefetched. And testing for zero happily succeeds. See also https://sourceware.org/pipermail/elfutils-devel/2021q3/004165.html
Fixed in commit 5beca45e639f153ed08109dc904cd9bc74e094e8. run-debuginfod-fd-prefetch-caches.sh now tests if the file descriptor or megabyte maxes are enforced.