Add KVGateway tests, fix cacheTtl validation and last cursor (#416)

- Upgrade to `ava@5`
- Port `KVNamespace` tests from Miniflare 2
- Only throw if `cacheTtl` is invalid, not just specified
- Return `cursor: undefined` on the last page to match the behaviour
  of the Worker's runtime

Author: mrbbot

ava.config.mjs

@@ -8,9 +8,6 @@ const rewritePaths = Object.fromEntries(
 );
 
 export default {
-  nonSemVerExperiments: {
-    nextGenConfig: true,
-  },
   files: ["packages/*/test/**/*.spec.ts"],
   timeout: "5m",
   nodeArguments: ["--no-warnings", "--experimental-vm-modules"],