Add Amazon.Lambda.DurableExecution.Testing design doc

Spec for a testing SDK companion to Amazon.Lambda.DurableExecution:
local DurableTestRunner with in-memory orchestration, CloudDurableTestRunner
for integration tests, step inspection API, sibling-function registration,
and safety limits. Adopts the IDurableServiceClient seam (Approach B) that
Python and JavaScript reference SDKs converged on.

Tidy testing SDK design — drop A/B/C exploration framing

The "Approach A vs B vs C" framing only made sense in the brainstorming
chat where the alternatives had been spelled out earlier. The committed
spec should present the chosen design directly and explain why the
service-client interface is right on its own merits.

Author: GarrettBeatty