selftests/bpf: Range analysis test case for JSET

This patch adds coverage for the warning detected by syzkaller and fixed
in the previous patch. Without the previous patch, this test fails with:

  verifier bug: REG INVARIANTS VIOLATION (false_reg1): range bounds
  violation u64=[0x0, 0x0] s64=[0x0, 0x0] u32=[0x1, 0x0] s32=[0x0, 0x0]
  var_off=(0x0, 0x0)(1)

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/c7893be1170fdbcf64e0200c110cdbd360ce7086.1752171365.git.paul.chaignon@gmail.com


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