selftests/bpf: Print reason when a tester could not run a program

Commit 8184d44c ("selftests/bpf: skip verifier tests for unsupported
program types") added a check to skip unsupported program types. As
bpf_probe_prog_type can change errno, do_single_test should save it before
printing a reason why a supported BPF program type failed to load.

Fixes: 8184d44c ("selftests/bpf: skip verifier tests for unsupported program types")
Signed-off-by: Florian Lehner <dev@der-flo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201204181828.11974-2-dev@der-flo.net