samples: pytest: improvements and fixes

[mniestroj]

All samples but DFU are not handlng properly the case when --initial-timeout is not provided (which is true for all platforms other than nRF91). request.config.getoption('--initial-timeout', DEFAULT_TIMEOUT) retured None, which means that None was passed to pexpect.expect() or pexpect.readline() methods. This was resulting in infinite timeouts, instead of default 30s timeouts.

Copy initial_timeout fixture from DFU's pytest script to all other samples' scripts.

Remove dead code in lightdb/observe pytest script.

Fix lightdb/get pytest script to handle counter values logged as hexdump. See commit message for details why this was not detected in CI (and why other bugs might be missed as well).

[github-actions[bot]]

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