`assert_iamframe_equals()` passes if all-nan-cols are present

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

Currently, assert_iamframe_equal() fails if one of the IamDataFrames has a meta-column of all-nan's (and the column doesn't exist in the other IamDataFrame). Arguably, all-nan should be treated as identical to "column doesn't exist".

The PR also adds a page for the testing module to the docs.

Closes #515

[codecov[bot]]

Codecov Report

Merging #608 (005434b) into main (28089d3) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #608   +/-   ##
=====================================
  Coverage   94.3%   94.3%           
=====================================
  Files         53      54    +1     
  Lines       5471    5477    +6     
=====================================
+ Hits        5162    5168    +6     
  Misses       309     309           

Impacted Files	Coverage Δ
pyam/testing.py	75.0% <100.0%> (ø)
tests/test_testing.py	100.0% <100.0%> (ø)

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[phackstock]

Perfect, good to merge from my side.