Syntax errors when importing plugin in 2018 Unity

[raslab]

General
2018th Unity needed assembly definitions for unit test scripts and all playmode tests cannot be compiled because of broken references to NUnit libraries.
Achieved in Unity version: 2018.4.6f1 (LTS)

STR:

1. Clone repository
2. Copy all files from REX-Diagnostics\RexWindowProject\Assets to some folder inside exists project
3. See syntax errors in log
   

Expected: Normal script compiling without errors

Way to fix:
Need to add 2 assembly definitions in projects - one for tests and one for all another

Tests assembly definitions need to be marked as Test assembly and referenced with main assembly definition