selftests/bpf: Avoid skipping certain subtests

selftests/bpf: Avoid skipping certain subtests

Commit 94f361ea4c7b ('selftests/bpf: Test "bpftool gen min_core_btf"')
added a test test_core_btfgen to test core relocation with btf
generated with 'bpftool gen min_core_btf'. Currently,
among 76 subtests, 25 are skipped.

  ...
  #46/69   core_reloc_btfgen/enumval:OK
  #46/70   core_reloc_btfgen/enumval___diff:OK
  #46/71   core_reloc_btfgen/enumval___val3_missing:OK
  #46/72   core_reloc_btfgen/enumval___err_missing:SKIP
  #46/73   core_reloc_btfgen/enum64val:OK
  #46/74   core_reloc_btfgen/enum64val___diff:OK
  #46/75   core_reloc_btfgen/enum64val___val3_missing:OK
  #46/76   core_reloc_btfgen/enum64val___err_missing:SKIP
  ...
  #46      core_reloc_btfgen:SKIP
  Summary: 1/51 PASSED, 25 SKIPPED, 0 FAILED

Alexei found that in the above core_reloc_btfgen/enum64val___err_missing
should not be skipped.

Currently, the core_reloc tests have some negative tests.
In Commit 94f361ea4c7b, for core_reloc_btfgen, all negative tests
are skipped with the following condition
  if (!test_case->btf_src_file || test_case->fails) {
	test__skip();
	continue;
  }
This is too conservative. Negative tests do not fail
mkstemp() and run_btfgen() should not be skipped.
There are a few negative tests indeed failing run_btfgen()
and this patch added 'run_btfgen_fails' to mark these tests
so that they can be skipped for btfgen tests. With this,
we have
  ...
  #46/69   core_reloc_btfgen/enumval:OK
  #46/70   core_reloc_btfgen/enumval___diff:OK
  #46/71   core_reloc_btfgen/enumval___val3_missing:OK
  #46/72   core_reloc_btfgen/enumval___err_missing:OK
  #46/73   core_reloc_btfgen/enum64val:OK
  #46/74   core_reloc_btfgen/enum64val___diff:OK
  #46/75   core_reloc_btfgen/enum64val___val3_missing:OK
  #46/76   core_reloc_btfgen/enum64val___err_missing:OK
  ...
  Summary: 1/62 PASSED, 14 SKIPPED, 0 FAILED

Totally 14 subtests are skipped instead of 25.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220614055526.628299-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>