tests: Don't fail run-debuginfod-fd-prefetch-caches.sh if grep -c fails

tests: Don't fail run-debuginfod-fd-prefetch-caches.sh if grep -c fails

The set -o errtrace made run-debuginfod-fd-prefetch-caches.sh
fail. On some systems. Add set -o functrace to make it fail consistently.

The failure is because the grep -c for in the log file fails (it
returns zero). Fix this by using || true. But this is only a
workaround. It makes the test pass, but only because all values are
always zero. The test doesn't currently test anything.

Also make sure that err and cleanup are only executed once.

Signed-off-by: Mark Wielaard <mark@klomp.org>