Change mockKV() to createKV()

[kulla]

The current way to mock a KV store is

https://github.com/serlo/serlo.org-cloudflare-worker/blob/c96e71181f854840b335edb1118add87e3917042/__tests__/_helper.ts#L64-L77

The function adds a mock for the KV to the global namespace. With this approach we need a generic variable setting at https://github.com/serlo/serlo.org-cloudflare-worker/blob/c96e71181f854840b335edb1118add87e3917042/__tests__/_helper.ts#L67 and the line https://github.com/serlo/serlo.org-cloudflare-worker/blob/c96e71181f854840b335edb1118add87e3917042/__tests__/_helper.ts#L64 is a duplication of the definition at https://github.com/serlo/serlo.org-cloudflare-worker/blob/c96e71181f854840b335edb1118add87e3917042/src/kv.d.ts#L22-L28

A solution is to create a function createKV() at __tests__/__utils__/index.ts with the signature:

export function createKV<Keys extends string>(): KV<Keys> {
   ...
}

Then we can use this function in our test code with for example

global.PATH_INFO_KV = createKV()

See https://developers.cloudflare.com/workers/runtime-apis/kv for a doc what a KV store is.