tests: avoid using the journalctl cursor when it has not been created yet

[sergiocazzolato]

This is for the scenario when the cursor was not calculated and the test
suite fails on the first test during the preparation.

This is the error we want to fix:
https://paste.ubuntu.com/p/RJH7cZXSwT/

[chipaca]

nice

[codecov-io]

Codecov Report

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@@            Coverage Diff            @@
##             master    #5662   +/-   ##
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  Coverage          ?   78.95%           
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  Files             ?      521           
  Lines             ?    39682           
  Branches          ?        0           
=========================================
  Hits              ?    31331           
  Misses            ?     5805           
  Partials          ?     2546

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