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.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231214225016.1209867-3-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>