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.cc

@@ -0,0 +1,52 @@
+#include <regex>
+
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
+
+#include "tests/derived-path.hh"
+#include "tests/libstore.hh"
+
+namespace rc {
+using namespace nix;
+
+Gen<DerivedPath> Arbitrary<DerivedPath>::arbitrary()
+{
+    switch (*gen::inRange<uint8_t>(0, 1)) {
+    case 0:
+        return gen::just((DerivedPath) DerivedPath::Opaque {
+            .path = *gen::arbitrary<StorePath>(),
+        });
+    default:
+        return gen::just((DerivedPath) DerivedPath::Built {
+            .drvPath = *gen::arbitrary<StorePath>(),
+            .outputs = *gen::arbitrary<OutputsSpec>(),
+        });
+    }
+}
+
+}
+
+namespace nix {
+
+class DerivedPathTest : public LibStoreTest
+{
+};
+
+// FIXME: `RC_GTEST_FIXTURE_PROP` isn't calling `SetUpTestSuite` because it is
+// no a real fixture.
+//
+// See https://github.com/emil-e/rapidcheck/blob/master/doc/gtest.md#rc_gtest_fixture_propfixture-name-args
+TEST_F(DerivedPathTest, force_init)
+{
+}
+
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathTest,
+    prop_round_rip,
+    (const DerivedPath & o))
+{
+    RC_ASSERT(o == DerivedPath::parse(*store, o.to_string(*store)));
+}
+
+}