selftests/bpf: fix timer/test_bad_ret subtest on test_progs-cpuv4 flavor

selftests/bpf: fix timer/test_bad_ret subtest on test_progs-cpuv4 flavor

Because test_bad_ret main program is not written in assembly, we don't
control instruction indices in timer_cb_ret_bad() subprog. This bites us
in timer/test_bad_ret subtest, where we see difference between cpuv4 and
other flavors.

For now, make __msg() expectations not rely on instruction indices by
anchoring them around bpf_get_prandom_u32 call. Once we have regex/glob
support for __msg(), this can be expressed a bit more nicely, but for
now just mitigating the problem with available means.

Fixes: e02dea158dda ("selftests/bpf: validate async callback return value check correctness")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231208233028.3412690-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>