Improve sync test linked to network availability & fix Post request response

[opatry]

Description

Relying on "request count" in test to determine which should be a "no network connection" or not wasn't ideal.
Making things explicit is better.
Also, the Post request was returning a response list but should have been a single TaskList.

Note: this is still a bit weird/fragile to rely on endpoint url & Http verb to determine the result.
Typically, Get task list here always return empty, but wouldn't after second sync() call.

This is left for later if a better pattern emerges.

Checklist

• I have read the CONTRIBUTING guide
• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing