Add test for set_attributes

[wiegandm]

This commit adds a test for the last remaining untested method of the
XmlCommandElement class, set_attributes.

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master       #5     +/-   ##
=========================================
+ Coverage   12.56%   12.66%   +0.1%     
=========================================
  Files           9        9             
  Lines        1974     1974             
=========================================
+ Hits          248      250      +2     
+ Misses       1726     1724      -2

Impacted Files	Coverage Δ
gvm/xml.py	18.51% <0%> (+0.14%)	⬆️

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