selftests/bpf: Adding delay in socketmap_listen to reduce flakyness

This patch adds a 1ms delay to reduce flakyness of the test.

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210819163609.2583758-1-fallentree@fb.com