Automated testing doesn't work properly with disabled WiFi and charging (iOS)

[sloncocs]

Describe the bug
With Run tests automatically enabled and Only while charging and Only on WiFi settings disabled, the interface updates the last automated test and number of automated tests only once in a few days.

To Reproduce

1. Enable Run tests automatically option on iOS
2. Disable Only while charging and Only on WiFi settings
3. Wait 3-4 days for the app to run tests in background mode
4. See the date is updated only after several dates and the updated number of tests (approximately 2-3 per day)

Expected behavior
The tests should be ran hourly, the date and number of the tests ran should be updated on the app accordingly.

Screenshots

The date and number of tests shown on September 5th at 14:53

The date and number of tests shown on September 6th at 10:37

System information (please complete the following information):

• Device: iPhoneXR
• OS: iOS14.5.1
• OONI Probe version: 3.7.0

Additional context

On iOS the background task can be suspended by the system and continues when the system thinks there is enough resources for it to continue.
If we are looking at only the results, that would not capture the whole picture. The measurement can be scheduled at the right time but only completes in a future date later than the expected runtime.

[aanorbel]

This issue has multiple background processing-related concerns.
For now, the issue will be observed with the changes to background processing which interruptTest in the task expiration handler.