fix: Handle first timestamp in deployment logs more carefully

[vpodzime]

Depending on the system and build configuration, the timestamps in logs can use lower (likely) or higher (unlikely) time resolution than expected (nanoseconds). So in case of a deployment failure with corrupted logs, when using the first timestamp as a replacement in the extra (THE ORIGINAL LOGS CONTAINED INVALID ENTRIES) log entry, care must be taken to make sure the extra log entry still has a valid timestamp and that the result is valid JSON.

Ticket: MEN-9427
Changelog: commit

Steps:

• code fix
• new unit tests
• new integration test

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[mender-test-bot]

Merging these commits will result in the following changelog entries:

Changelogs

mender (master-bad_depl_log_tstamp)

New changes in mender since master:

Bug Fixes

• Handle first timestamp in deployment logs more carefully

  Depending on the system and build configuration, the timestamps
  in logs can use lower (likely) or higher (unlikely) time
  resolution than expected (nanoseconds). So in case of a
  deployment failure with corrupted logs, when using the first
  timestamp as a replacement in the extra (THE ORIGINAL LOGS CONTAINED INVALID ENTRIES) log entry, care must be taken to make
  sure the extra log entry still has a valid timestamp and that the
  result is valid JSON.
  (MEN-9427)