Split data source tests to a separate workflow

[khaeru]

Currently these tests in sequence and can be very slow; several hours, e.g. here. In this PR:

• Split the service tests to a separate workflow file.
• Run each source's tests in a separate job, in parallel.
• Add a job to collect the results and compile them.
• Use the term "source" consistently throughout.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

Merging #131 (2f3edfb) into main (f0af35a) will decrease coverage by 0.04%.
The diff coverage is 18.75%.

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   95.83%   95.80%   -0.04%     
==========================================
  Files          74       74              
  Lines        5619     5622       +3     
==========================================
+ Hits         5385     5386       +1     
- Misses        234      236       +2     

Impacted Files	Coverage Δ
sdmx/model/internationalstring.py	100.00% <ø> (ø)
sdmx/tests/test_sources.py	67.37% <0.00%> (-0.37%)	⬇️
sdmx/testing/report.py	43.90% <33.33%> (+0.31%)	⬆️