selftests/bpf: add file dynptr tests

selftests/bpf: add file dynptr tests

Introducing selftests for validating file-backed dynptr works as
expected.
 * validate implementation supports dynptr slice and read operations
 * validate destructors should be paired with initializers
 * validate sleepable progs can page in.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251026203853.135105-11-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>