Fix: OONIAPIClientTest using failing API endpoint and incorrect testing logic

[sdsantos]

Fixes ooni/probe#2778

Proposed Changes

• Switch testing endpoint from ams-pg.ooni.org to https://backend-hel.ooni.org
• Refactor OONIAPIClientTest to Kotlin
• Execute API calls synchronously to fix incorrect async logic
• Remove database operations that were irrelevant to the testSelectMeasurementsWithJson test
• Remove the test getMeasurementJsonError because it was only testing external libraries logic (retrofit and gson) and not the OONIAPIClient

[aanorbel]

🐳