timer: add unit tests for DeferReactivation

timer: add unit tests for DeferReactivation

Create a unit test for systemd timer DeferReactivation config option.
The test works by creating a timer which fires every 5 seconds and
starts an unit which runs for 5 seconds.
With DeferReactivation=true, the timer must fire every 5+5 seconds,
instead of the 5 it fires normally.

As we need at least two timer runs to check if the delta is correct,
the test duration on success will be at least 20 seconds.
To be safe, the test script waits 35 seconds: this is enough to get
at least three runs but low enough to avoid clogging the CI.