selftests/bpf: Make reg_bounds test more robust

The verifier log output may contain multiple lines that start with
18: (bf) r0 = r6
teach reg_bounds to look for lines that have ';' in them,
since reg_bounds test is looking for:
18: (bf) r0 = r6       ; R0=... R6=...

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260325012242.45606-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>