selftests/bpf: test if state loops are detected in a tricky case

A convoluted test case for iterators convergence logic that
demonstrates that states with branch count equal to 0 might still be
a part of not completely explored loop.

E.g. consider the following state diagram:

               initial     Here state 'succ' was processed first,
                 |         it was eventually tracked to produce a
                 V         state identical to 'hdr'.
    .---------> hdr        All branches from 'succ' had been explored
    |            |         and thus 'succ' has its .branches == 0.
    |            V
    |    .------...        Suppose states 'cur' and 'succ' correspond
    |    |       |         to the same instruction + callsites.
    |    V       V         In such case it is necessary to check
    |   ...     ...        whether 'succ' and 'cur' are identical.
    |    |       |         If 'succ' and 'cur' are a part of the same loop
    |    V       V         they have to be compared exactly.
    |   succ <- cur
    |    |
    |    V
    |   ...
    |    |
    '----'

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231024000917.12153-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>