Fix: BeforeTestRun not run in .NET462 up to .NET481 in multitarget test project (#146)

[gasparnagy]

Fixes #146

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue).

Checklist:

• [x] I've added tests for my code. (most of the time mandatory)
• [x] I have added an entry to the changelog. (mandatory)

[gasparnagy]

The good news is that after I enabled this in system tests, only xunit caused a problem, NUnit worked.