record start_time property workaround

[rplevka]

This is a workaround for:
pytest-dev/pytest#7767

where the pre-defined record_testsuite_timestamp_xml fixture doesn't work with xdist yet.
This commit keeps the fixture, but sets autouse=False

and implements a manual property recording inside the report_header hook

[tstrych]

LGTM
ACK

[rplevka]

thanks @tstrych for discussing the .now() vs .utcnow(). After reviewing the code I realized we actually use .utcnow() in the logger too, so I reverted it back just as you suggested.

[codecov]

Codecov Report

Merging #8122 into master will decrease coverage by 0.12%.
The diff coverage is 48.74%.

@@            Coverage Diff             @@
##           master    #8122      +/-   ##
==========================================
- Coverage   58.56%   58.44%   -0.13%     
==========================================
  Files          74       77       +3     
  Lines        5795     6235     +440     
==========================================
+ Hits         3394     3644     +250     
- Misses       2401     2591     +190     

Impacted Files	Coverage Δ
robottelo/api/assertions.py	100.00% <ø> (ø)
robottelo/cli/activationkey.py	53.48% <ø> (-1.06%)	⬇️
robottelo/cli/architecture.py	100.00% <ø> (ø)
robottelo/cli/assertions.py	100.00% <ø> (ø)
robottelo/cli/computeresource.py	54.83% <ø> (+10.83%)	⬆️
robottelo/cli/contentview.py	47.74% <ø> (+4.61%)	⬆️
robottelo/cli/discoveryrule.py	100.00% <ø> (ø)
robottelo/cli/domain.py	100.00% <ø> (ø)
robottelo/cli/environment.py	71.42% <ø> (-3.58%)	⬇️
robottelo/cli/filter.py	71.42% <ø> (-3.58%)	⬇️
... and 89 more

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