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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/progs/timer_failure.c

···
 __flag(BPF_F_TEST_STATE_FREQ)
 __failure
 /* check that fallthrough code path marks r0 as precise */
-__msg("mark_precise: frame0: regs=r0 stack= before 22: (b7) r0 = 0")
+__msg("mark_precise: frame0: regs=r0 stack= before")
+__msg(": (85) call bpf_get_prandom_u32#7") /* anchor message */
 /* check that branch code path marks r0 as precise */
-__msg("mark_precise: frame0: regs=r0 stack= before 24: (85) call bpf_get_prandom_u32#7")
+__msg("mark_precise: frame0: regs=r0 stack= before ") __msg(": (85) call bpf_get_prandom_u32#7")
 __msg("should have been in [0, 0]")
 long BPF_PROG2(test_bad_ret, int, a)
 {