Split up test project

Robust.UnitTesting was both ALL tests for RT, and also API surface for content tests.

Tests are now split into separate projects as appropriate, and the API side has also been split off.

Robust.Shared.Maths.Tests/Vector2u_Test.cs

@@ -0,0 +1,20 @@
+using System.Text.Json;
+using NUnit.Framework;
+
+namespace Robust.Shared.Maths.Tests
+{
+    [TestFixture]
+    [Parallelizable]
+    [TestOf(typeof(Vector2u))]
+    internal sealed class Vector2u_Test
+    {
+        // This test basically only exists because RSI loading needs it.
+        [Test]
+        public void TestJsonDeserialization()
+        {
+            Assert.That(
+                JsonSerializer.Deserialize<Vector2u>("{\"x\": 10, \"y\": 10}",
+                    new JsonSerializerOptions(JsonSerializerDefaults.Web)), Is.EqualTo(new Vector2u(10, 10)));
+        }
+    }
+}