Dedup some derivation initialization logic, and test

`nix derivation add`, and its C API counterpart, now works a bit closer
to `builtins.derivation` in that they don't require the user to fill-in
input addressed paths correctly ahead of time.

The logic for this is carefully deduplicated, between all 3 entry
points, and also between the existing `checkInvariants` function. There
are some more functional tests, and there are also many more unit tests.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>
Co-authored-by: edef <edef@edef.eu>

src/libstore-tests/derivation/test-support.hh

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <gtest/gtest.h>
+
+#include "nix/util/experimental-features.hh"
+#include "nix/store/tests/libstore.hh"
+#include "nix/util/tests/characterization.hh"
+
+namespace nix {
+
+class DerivationTest : public virtual CharacterizationTest, public LibStoreTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "derivation";
+
+public:
+    std::filesystem::path goldenMaster(std::string_view testStem) const override
+    {
+        return unitTestData / testStem;
+    }
+
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+};
+
+class CaDerivationTest : public DerivationTest
+{
+    void SetUp() override
+    {
+        mockXpSettings.set("experimental-features", "ca-derivations");
+    }
+};
+
+class DynDerivationTest : public DerivationTest
+{
+    void SetUp() override
+    {
+        mockXpSettings.set("experimental-features", "dynamic-derivations ca-derivations");
+    }
+};
+
+class ImpureDerivationTest : public DerivationTest
+{
+    void SetUp() override
+    {
+        mockXpSettings.set("experimental-features", "impure-derivations");
+    }
+};
+
+} // namespace nix