selftests/bpf: Only run tests if !bpf_disabled

The "kernel.unprivileged_bpf_disabled" sysctl, if enabled, causes all
unprivileged tests to fail because it permanently disables unprivileged
BPF access for the currently running kernel. Skip the relevant tests if
the user attempts to run the testsuite with this sysctl enabled.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>