selftests/bpf: Don't check for btf fd in test_btf

selftests/bpf: Don't check for btf fd in test_btf

After commit 0d13bfce023a ("libbpf: Don't require root for
bpf_object__open()") we no longer load BTF during bpf_object__open(),
so let's remove the expectation from test_btf that the fd is not -1.
The test currently fails.

Before:
BTF libbpf test[1] (test_btf_haskv.o): do_test_file:4152:FAIL bpf_object__btf_fd: -1
BTF libbpf test[2] (test_btf_newkv.o): do_test_file:4152:FAIL bpf_object__btf_fd: -1
BTF libbpf test[3] (test_btf_nokv.o): do_test_file:4152:FAIL bpf_object__btf_fd: -1

After:
BTF libbpf test[1] (test_btf_haskv.o): OK
BTF libbpf test[2] (test_btf_newkv.o): OK
BTF libbpf test[3] (test_btf_nokv.o): OK

Fixes: 0d13bfce023a ("libbpf: Don't require root for bpf_object__open()")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200118010546.74279-1-sdf@google.com