selftests/bpf: Utilize string values for delegate_xxx mount options

Use both hex-based and string-based way to specify delegate mount
options for BPF FS.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-21-andrii@kernel.org