selftests/bpf: Fix core_reloc test runner

Fix failed tests checks in core_reloc test runner, which allowed failing tests
to pass quietly. Also add extra check to make sure that expected to fail test cases with
invalid names are caught as test failure anyway, as this is not an expected
failure mode. Also fix mislabeled probed vs direct bitfield test cases.

Fixes: 124a892d ("selftests/bpf: Test TYPE_EXISTS and TYPE_SIZE CO-RE relocations")
Reported-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Lorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210426192949.416837-6-andrii@kernel.org