Fix MVIRI tests with unstable dependencies

[sfinkens]

Convert scanline timestamps to nanoseconds before averaging them. Looks like a bug in xarray (pydata/xarray#9975), but converting to nanosecond doesn't hurt

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (01237e2) to head (b4abff1).
Report is 53 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3041   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files         383      383           
  Lines       55673    55674    +1     
=======================================
+ Hits        53511    53512    +1     
  Misses       2162     2162           

Flag	Coverage Δ
behaviourtests	3.89% <0.00%> (-0.01%)	⬇️
unittests	96.21% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 12912700425

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.222%

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Totals
Change from base Build 12905542942:	0.0%
Covered Lines:	53759
Relevant Lines:	55870

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💛 - Coveralls