[tests] Add a global.json targeting dotnet 3.1.100 to the dotnet unit tests. (#8728)

Because VSfM isn't able to build this project otherwise:
https://developercommunity.visualstudio.com/content/problem/1056720/cant-build-net-5-project-error-net-core-31-sdk-is.html

tests/dotnet/UnitTests/global.json

@@ -0,0 +1,3 @@
+{
+	"sdk": { "version": "3.1.100" }
+}