libbpf: Selftests for resizing datasec maps

This patch adds test coverage for resizing datasec maps. The first two
subtests resize the bss and custom data sections. In both cases, an
initial array (of length one) has its element set to one. After resizing
the rest of the array is filled with ones as well. A BPF program is then
run to sum the respective arrays and back on the userspace side the sum
is checked to be equal to the number of elements.
The third subtest attempts to perform resizing under conditions that
will result in either the resize failing or the BTF info being cleared.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230524004537.18614-3-inwardvessel@gmail.com