selftests/bpf: Fix selftests broken by mitigations=off

When we configure the kernel command line with 'mitigations=off' and set
the sysctl knob 'kernel.unprivileged_bpf_disabled' to 0, the commit
bc5bc309 ("bpf: Inherit system settings for CPU security mitigations")
causes issues in the execution of `test_progs -t verifier`. This is
because 'mitigations=off' bypasses Spectre v1 and Spectre v4 protections.

Currently, when a program requests to run in unprivileged mode
(kernel.unprivileged_bpf_disabled = 0), the BPF verifier may prevent
it from running due to the following conditions not being enabled:

  - bypass_spec_v1
  - bypass_spec_v4
  - allow_ptr_leaks
  - allow_uninit_stack

While 'mitigations=off' enables the first two conditions, it does not
enable the latter two. As a result, some test cases in
'test_progs -t verifier' that were expected to fail to run may run
successfully, while others still fail but with different error messages.
This makes it challenging to address them comprehensively.

Moreover, in the future, we may introduce more fine-grained control over
CPU mitigations, such as enabling only bypass_spec_v1 or bypass_spec_v4.

Given the complexity of the situation, rather than fixing each broken test
case individually, it's preferable to skip them when 'mitigations=off' is
in effect and introduce specific test cases for the new 'mitigations=off'
scenario. For instance, we can introduce new BTF declaration tags like
'__failure__nospec', '__failure_nospecv1' and '__failure_nospecv4'.

In this patch, the approach is to simply skip the broken test cases when
'mitigations=off' is enabled. The result of `test_progs -t verifier` as
follows after this commit,

Before this commit
==================

- without 'mitigations=off'
  - kernel.unprivileged_bpf_disabled = 2
    Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
  - kernel.unprivileged_bpf_disabled = 0
    Summary: 74/1336 PASSED, 0 SKIPPED, 0 FAILED    <<<<
- with 'mitigations=off'
  - kernel.unprivileged_bpf_disabled = 2
    Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
  - kernel.unprivileged_bpf_disabled = 0
    Summary: 63/1276 PASSED, 0 SKIPPED, 11 FAILED   <<<< 11 FAILED

After this commit
=================

- without 'mitigations=off'
  - kernel.unprivileged_bpf_disabled = 2
    Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
  - kernel.unprivileged_bpf_disabled = 0
    Summary: 74/1336 PASSED, 0 SKIPPED, 0 FAILED    <<<<
- with this patch, with 'mitigations=off'
  - kernel.unprivileged_bpf_disabled = 2
    Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED
  - kernel.unprivileged_bpf_disabled = 0
    Summary: 74/948 PASSED, 388 SKIPPED, 0 FAILED   <<<< SKIPPED

Fixes: bc5bc309

 ("bpf: Inherit system settings for CPU security mitigations")
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Closes: https://lore.kernel.org/bpf/CAADnVQKUBJqg+hHtbLeeC2jhoJAWqnmRAzXW3hmUCNSV9kx4sQ@mail.gmail.com
Link: https://lore.kernel.org/bpf/20231025031144.5508-1-laoar.shao@gmail.com