Some test consolidation

[mrbean-bremen]

• fixed possibility to run single tests
• use full standard instead of fixture files for most tests
• download and cache standard in the CI
• fix to handle empty tag with type 1C as an error
• use lxml used instead of xml for xml parsing

[codecov-commenter]

Codecov Report

Merging #56 (e163b5d) into main (90207ba) will increase coverage by 0.21%.
Report is 3 commits behind head on main.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   88.71%   88.93%   +0.21%     
==========================================
  Files          14       14              
  Lines        1515     1518       +3     
==========================================
+ Hits         1344     1350       +6     
+ Misses        171      168       -3     

Files	Coverage Δ
dicom_validator/spec_reader/edition_reader.py	98.45% <100.00%> (ø)
dicom_validator/validator/iod_validator.py	92.34% <100.00%> (-0.82%)	⬇️
dicom_validator/spec_reader/spec_reader.py	94.64% <50.00%> (+0.30%)	⬆️

... and 4 files with indirect coverage changes

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