Auto-update signed definition timestamp at firmware release time

[matejcik]

leftover from #2410:

we need a script that will, presumably:

1. download the latest published signed definitions
2. extract timestamp
3. update released-definitions-timestamp.txt

there is no point in implementing that now before any signed definitions exist.

as a part of this task, testcases that download the current signed definitions should be implemented

[mmilata]

This was implemented in 10b248b18fe807cefd93ac373743448a4b235f5a except the testcases part.