tests/run-debuginfod-fd-prefetch-caches.sh: disable fdcache limit check

tests/run-debuginfod-fd-prefetch-caches.sh: disable fdcache limit check

Since commit acd9525e93d7 ("PR31265 - rework debuginfod archive-extract
fdcache"), the fdcache limit is only applied when a new file is interned
and it has been at least 10 seconds since the limit was last applied.
This means that the fdcache can go over the limit temporarily.

run-debuginfod-fd-prefetch-caches.sh happens to avoid tripping over this
because of lucky sizes of the files used in the test.  However, adding
new files for an upcoming test exposed this failure.

Disable this part of the test for now.

Signed-off-by: Omar Sandoval <osandov@fb.com>