tests: Fix normalization of nat_timeused in prompt in journal

tests: Fix normalization of nat_timeused in prompt in journal

Command prompts show nat_timeused rounded down to minutes.  They need
to be normalized, or else tests can fail when they take too long, or
cross midnight.  Formatted prompts are normalized correctly (not
actually used since commit 9ca3fa9), but the journal contains raw
prompts.  Normalize them, too.

Smoke test's journal.log is affected, because the server charges at
least 15s per login, which adds up into minutes in this test.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>