selftests/bpf: Update reg_bound range refinement logic

selftests/bpf: Update reg_bound range refinement logic

This patch updates the range refinement logic in the reg_bound test to
match the new logic from the previous commit. Without this change, tests
would fail because we end with more precise ranges than the tests
expect.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/b7f6b1fbe03373cca4e1bb6a113035a6cd2b3ff7.1753695655.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>