Move fixtures directory and clean up locator

test/PathHelper.cs

@@ -0,0 +1,22 @@
+using System.Diagnostics;
+
+public static class PathHelper
+{
+    public static string GetFixturesDir() 
+        => Path.Combine(GetTestRoot(), "fixtures");
+
+    public static string GetFixture(params string[] names) 
+        => Path.Combine(new string[] { GetTestRoot(), "fixtures" }.Concat(names).ToArray());
+
+    public static string GetTestRoot()
+    {
+        // XXX: This is an evil hack, but it's okay for a unit test
+        // We can't use Assembly.Location because unit test runners love
+        // to move stuff to temp directories
+        var st = new StackFrame(true);
+#pragma warning disable CS8604 // Possible null reference argument.
+        var di = new DirectoryInfo(Path.Combine(Path.GetDirectoryName(st.GetFileName())));
+#pragma warning restore CS8604 // Possible null reference argument.
+        return di.FullName;
+    }
+}