refactor: improve generator-cli test performance

[shojisoma]

Closes https://github.com/SAP/cloud-sdk-backlog/issues/282

Changes

1. use mock-fs to improve the performance when the test actually writes files to the repo.
2. test generte() directly to use mock-fs instead of executing the cli with execa because execa and mock-fs are running on different processes hence these cannot use on the same process.
3. move createOptionsFromConfig() into generator-options.ts in order to test the function directly.