Moved sourcecode tests to separate tool

[304NotModified]

• Removes false positives in code covage
• No need to run 3 times
• Clearer errors
• No need to use xUnit or nUnit for this
• Prepared for usage in other repositories

[codecov-io]

Current coverage is 72.91%

Merging #1043 into master will decrease coverage by -0.42% as of 3801913

@@            master   #1043   diff @@
======================================
  Files          263     263       
  Stmts        14873   14873       
  Branches      1617    1617       
  Methods          0       0       
======================================
- Hit          10907   10845    -62
  Partial        416     416       
- Missed        3550    3612    +62

Review entire Coverage Diff as of 3801913

---

Uncovered Suggestions

1. +0.09% via ...argets/FileTarget.cs#1918...1930
2. +0.09% via ...argets/FileTarget.cs#1555...1567
3. +0.08% via ...c/NLog/LogFactory.cs#180...191
4. See 7 more...

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