PSA: test operations with small buffers

[gilles-peskine-arm]

In the PSA tests, we aren't systematically testing behavior when an output buffer is too small.

When an output buffer is too small:

• The function must return PSA_ERROR_BUFFER_TOO_SMALL (at least if no other error condition applies).
• The length output parameter must be set to a value that's smaller than the size.
• The output buffer should still be set to a safe value.

(Re-raised from a private tracker from the early days of the PSA code.)

Size estimate: between M and L. I'm filing as L because this is definitely not a single-PR task. There's not much of a design space but quite a bit of code to write.