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.UnitTesting/RobustIntegrationTestDummy.cs

@@ -0,0 +1,13 @@
+using NUnit.Framework;
+
+namespace Robust.UnitTesting;
+
+[TestFixture]
+internal sealed class RobustIntegrationTestDummy : RobustIntegrationTest
+{
+    [Test]
+    public void Foo()
+    {
+        // Nada.
+    }
+}