The solution was to set up the nuget.castle-core package. In this case, nuget.moq was setup correctly, so in the included test project, Tests.asmdef looks like this:

{
...
"precompiledReferences": [
"Moq.dll",
"nunit.framework.dll",
"System.Runtime.CompilerServices.Unsafe.dll",
"System.Threading.Tasks.Extensions.dll"
],
...
}

But nuget.moq depends on nuget.castle-core, which also needs to be set up (https://docs.unity3d.com/Packages/nuget.castle-core@2.0/manual/index.html#setup).

So the Tests.asmdef should look like:

{
...
"precompiledReferences": [
"Castle.Core.dll",
"System.Diagnostics.EventLog.dll",
"Moq.dll",
"nunit.framework.dll",
"System.Runtime.CompilerServices.Unsafe.dll",
"System.Threading.Tasks.Extensions.dll"
],
...
}

With those changes (already documented), there should no longer be a problem.