selftests/bpf: Fix running of XDP bonding tests

An "innocent" cleanup in the last version of the XDP bonding patchset moved
the "test__start_subtest" calls to the test main function, but I forgot to
reverse the condition, which lead to all tests being skipped. Fix it.

Fixes: 6aab1c81

 ("selftests/bpf: Add tests for XDP bonding")
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210811123627.20223-1-joamaki@gmail.com