Add testutils module with assert_eq()

spec/testutils.d

@@ -0,0 +1,10 @@
+import std.stdio;
+
+void assert_eq(T)(T expected, T actual, string file = __FILE__, size_t line = __LINE__)
+{
+    if (expected != actual)
+    {
+        stderr.writeln(file, ":", line, ": expected ", expected, ", got ", actual);
+        assert(false);
+    }
+}