signindex: add JSON check for entry.json
This adds the JSON/SHA-256 check before signing for the index-v2 files, then adds a whole set of tests for that.
Ran this to generate/refresh the test index files:
cd tests; ../fdroid update --nosign --pretty
- converts config.py description to a single line, since the values are no longer stripped, so this give the same output.
closes #835 (closed)