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/path.hh

@@ -1,12 +1,25 @@
 #pragma once
 
-#include <rapidcheck.h>
+#include <rapidcheck/gen/Arbitrary.h>
 
-#include "path.hh"
+#include <path.hh>
+
+namespace nix {
+
+struct StorePathName {
+    std::string name;
+};
+
+}
 
 namespace rc {
 using namespace nix;
 
+template<>
+struct Arbitrary<StorePathName> {
+    static Gen<StorePathName> arbitrary();
+};
+
 template<>
 struct Arbitrary<StorePath> {
     static Gen<StorePath> arbitrary();