bpf/tests: Add JMP tests with small offsets

This patch adds a set of tests for JMP to verify that the JITed jump
offset is calculated correctly. We pretend that the verifier has inserted
any zero extensions to make the jump-over operations JIT to one
instruction each, in order to control the exact JITed jump offset.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-10-johan.almbladh@anyfinetworks.com