Port fixes for 3.16 release to the 4.0 development code

[CharliePoole]

The following issues and PRs, which were fixed for the 3.16, 3.16.1 and 3.16.2 releases, need to have corresponding changes made in the main branch for use with version 4.0. This will be done in stages, with multiple PRs. The issues are all labeled PortToVersion4 - the label to be removed as they are completed here.

• Error running tests from assembly built using VS2017 csproj file format #291
• Attempting to target too low a framework throws exception #299
• Allow netcoreapp agent to work with non-standard dotnet install locations #941
• Reorganize build output (bin) directories #1118
• v3.14 fails with target framework net461 ... net48 on linux (ArgumentException: The net-4.6.1 framework is not available.), but v3.12 works well  #1130
• Running tests using the VS Solution results in an exception if any projects target .NET Standard #1182
• Build hangs when test spawns processes which do not terminate properly #1183
• NUnit Console 3.15.2: Could not load file or assembly "System.Windows.Forms" #1203
• Add .NET 7.0 Agent #1216
• Assembly loading deduplication #1217
• Change name of NUnit Net Core Runner executable #1224
• Restore netcoreapp3.1 build to the engine package #1225
• Stop building console runner with .NET 2.0 #1232
• Upgrade Cake.Tool to 2.3.0 #1244
• Reduce build targets for nunit.engine.core assembly #1246
• Update .NET 7.0 Support from RC-2 to Final Release #1265
• Install dotnet tool failed #1271
• NUnit Console won't run unless .NET Desktop and ASP.NET Runtimes are installed #1274
• Missing assembly in NUnit.Engine nuget package #1275
• NUnit does not work without .NET Core  #1277
• Restore ability to debug packages #1284
• Use NUNIT_ prefix for API_KEY environment variables #1296