selftests/bpf: Add test cases for hash map with dynptr key

Add three positive test cases to test the basic operations on the
dynptr-keyed hash map. The basic operations include lookup, update,
delete and get_next_key. These operations are exercised both through
bpf syscall and bpf program. These three test cases use different map
keys. The first test case uses both bpf_dynptr and a struct with only
bpf_dynptr as map key, the second one uses a struct with an integer and
a bpf_dynptr as map key, and the last one use a struct with bpf_dynptr
being nested in another struct as map key.

Also add multiple negative test cases for dynptr-keyed hash map. These
test cases mainly check whether the layout of dynptr and non-dynptr in
the stack is matched with the definition of map->key_record.

Signed-off-by: Hou Tao <[email protected]>