More property tests

Also put proper comparison methods on `DerivedPath` and
`NixStringContextElem`, which is needed for the tests but good in
general.

src/libstore/tests/derived-path.hh

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <rapidcheck/gen/Arbitrary.h>
+
+#include <derived-path.hh>
+
+#include "tests/path.hh"
+#include "tests/outputs-spec.hh"
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<DerivedPath> {
+    static Gen<DerivedPath> arbitrary();
+};
+
+}