* Migrate tests to xunit

* Update sample project to dotnetcore3.1
* Use Sqlite in sample project to run it everywhere
* Fix: Filter with escaped comma
* Fix: Filter "null" does not work with Contains or StartsWith
* Code cleanup: Adjust namespaces, adjust usings

Sieve.Sample/appsettings.json

@@ -0,0 +1,22 @@
+{
+  "ConnectionStrings": {
+    "TestSqlServer": "Server=(localdb)\\MSSQLLocalDB; Database=SieveTests;"
+  },
+  "Sieve": {
+    "CaseSensitive": false,
+    "DefaultPageSize": 10
+  },
+  "Logging": {
+    "IncludeScopes": false,
+    "Debug": {
+      "LogLevel": {
+        "Default": "Warning"
+      }
+    },
+    "Console": {
+      "LogLevel": {
+        "Default": "Warning"
+      }
+    }
+  }
+}