[Fixes #6966] Thesauri intial data

[mattiagiupponi]

In the intial_data.json there were moked and invalid values for the thesauri.
With this PR i want to remove the moked values by:

• adding new value to the initial_data.json taken from the inspire-theme.rdf
• adding a new test_thesaurus.json in order to have specifi files required for the tests
• adding lang choice for MultiThesauriField

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[codecov]

Codecov Report

Merging #6967 (97e8f82) into master (7e1b7a0) will decrease coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #6967      +/-   ##
==========================================
- Coverage   58.27%   58.22%   -0.05%     
==========================================
  Files         552      552              
  Lines       38797    38801       +4     
  Branches     5227     5228       +1     
==========================================
- Hits        22610    22593      -17     
- Misses      14808    14826      +18     
- Partials     1379     1382       +3