Rename standard_name for various readers to be consistent

[djhoese]

Found out that AVHRR had incorrect standard_name from a mailing list issue. While fixing it I found that a couple readers had this issue.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[sfinkens]

LGTM, thanks for fixing this!

[coveralls]

Coverage increased (+0.01%) to 86.923% when pulling 905d2e4 on djhoese:bugfix-avhrr-standard-names into 8ded572 on pytroll:master.

[codecov]

Codecov Report

Merging #1006 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1006      +/-   ##
==========================================
+ Coverage   86.92%   86.92%   +<.01%     
==========================================
  Files         181      181              
  Lines       27553    27553              
==========================================
+ Hits        23950    23951       +1     
+ Misses       3603     3602       -1

Impacted Files	Coverage Δ
satpy/scene.py	90.53% <0%> (+0.17%)	⬆️

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[mraspaud]

LGTM