Debugging the NUnit3TestAdapter
Debugging the adapter is done by first creating a debug version of the adapter. You can then enable a debug run by passing one of the NUnit debug settings using runsettings.
The symbols are:
NUnit.DebugExecution
NUnit.DebugDiscovery
NUnit.Debug
The last setting is equal to setting both of the two above.
From command line, you can set these by adding
dotnet test -- NUnit.DebugExecution=true
If you want to do this using Visual Studio, you must add a runsettings file, and add these settings there to the NUnit section.
A detailed explanation of the process can be found in this blog post
Debugging earlier versions
See this blog post for details on that process.