More property tests

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

src/libexpr/tests/value/context.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <rapidcheck/gen/Arbitrary.h>
+
+#include <value/context.hh>
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<NixStringContextElem> {
+    static Gen<NixStringContextElem> arbitrary();
+};
+
+}