test_hs_descriptor: Add a test case that fails without the fix for 40793

This adds a bit more to hs_descriptor/test_decode_descriptor, mostly
testing pow-params and triggering the tor_assert() in issue #40793.

There was no mechanism for adding arbitrary test strings to the
encrypted portion of the desc without duplicating encode logic. One
option might be to publicize get_inner_encrypted_layer_plaintext enough
to add a mock implementation. In this patch I opt for what seems like
the simplest solution, at the cost of a small amount of #ifdef noise.
The unpacked descriptor grows a new test-only member that's used for
dropping arbitrary data in at encode time.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>