bpf: Always test unprivileged programs

If selftests are run as root, then execute the unprivileged checks as
well. This switch from 243 to 368 tests.

The test numbers are suffixed with "/u" when executed as unprivileged or
with "/p" when executed as privileged.

The geteuid() check is replaced with a capability check.

Handling capabilities requires the libcap dependency.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>