selftests/bpf: add randomized reg_bounds tests

Add random cases generation to reg_bounds.c and run them without
SLOW_TESTS=1 to increase a chance of BPF CI catching latent issues.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231112010609.848406-11-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>