chore: clarify fs routes testing

[ryux1]

What this PR does

Adds an info note to the route-testing guide that:

• .fsRoutes() consumes route data from Fresh's build cache rather than scanning routes/ during a direct unit test
• unit tests should register imported route handlers explicitly
• tests that need filesystem discovery, layouts, or islands should use the documented full Vite build path

This addresses the confusing all-404 behavior discussed in Q&A #3906.

Validation

• deno fmt --check docs/latest/testing/index.md
• deno task check:docs