selftests/bpf: Test BPF_LINK_UPDATE behavior for tracing links

Exercise a series of edge cases and happy path scenarios across a gamut
of link types (fentry, fmod_ret, fexit, freplace) to test
BPF_LINK_UPDATE behavior for tracing links. This test swaps
fentry/fmod_ret/fexit programs in-place and makes sure that the sequence
and attach cookie are as expected based on the link positions.

Signed-off-by: Jordan Rife <[email protected]>

Author: jrife