Add timezone to XCCDF TestResult start/end time
Add timezone to XCCDF TestResult start/end time

We add timezone information to the evaluation start date
and time using the format defined in XML specification
https://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#dateTime

We can't use strftime as strftime's %z modifier doesn't
put ':' (colon) between hours and minutes in timeshift
which is required by XML specification.

Fixes: #1449

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