net-test: bpf_test - deflake an assert

There's a very very small chance that prog_fd + 1 is already
occupied by some previously allocated file descriptor
(likely a leaked one...)

Bug: 304292059
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I81d99ab464cde734f4d5c2b8daa80227c2712ae1