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.Packaging.Tests/AssetPassTestCollector.cs

@@ -0,0 +1,52 @@
+using NUnit.Framework;
+using Robust.Packaging.AssetProcessing;
+
+namespace Robust.Packaging.Tests;
+
+/// <summary>
+/// A simple asset pass that stores all files it receives, for introspection by tests.
+/// </summary>
+public sealed class AssetPassTestCollector : AssetPass
+{
+    public readonly List<AssetFile> Files = [];
+
+    protected override AssetFileAcceptResult AcceptFile(AssetFile file)
+    {
+        lock (Files)
+        {
+            Files.Add(file);
+        }
+
+        return AssetFileAcceptResult.Consumed;
+    }
+
+    /// <summary>
+    /// Assert that the only files collected are an exact set of test files.
+    /// </summary>
+    public void AssertTextFiles(params (string path, string data)[] files)
+    {
+        lock (Files)
+        {
+            Assert.That(Files, Has.Count.EqualTo(files.Length));
+
+            Assert.Multiple(() =>
+            {
+                foreach (var file in files)
+                {
+                    var matchingFile = Files.SingleOrDefault(f => f.Path == file.path);
+                    if (matchingFile == null)
+                    {
+                        Assert.Fail($"Unable to find file {file.path}");
+                        continue;
+                    }
+
+                    using var fileData = matchingFile.Open();
+                    using var reader = new StreamReader(fileData);
+                    var fileText = reader.ReadToEnd();
+
+                    Assert.That(fileText, Is.EqualTo(file.data));
+                }
+            });
+        }
+    }
+}