Expose some core implementation details and write a basic unit test for the dummy store

This test currently doesn't use the new-exposed functionality, but with
future changes the tests will be expanded and they will be used.

src/libstore-tests/dummy-store.cc

@@ -0,0 +1,27 @@
+#include <gtest/gtest.h>
+
+#include "nix/store/dummy-store.hh"
+#include "nix/store/globals.hh"
+#include "nix/store/realisation.hh"
+
+namespace nix {
+
+TEST(DummyStore, realisation_read)
+{
+    initLibStore(/*loadConfig=*/false);
+
+    auto store = [] {
+        auto cfg = make_ref<DummyStoreConfig>(StoreReference::Params{});
+        cfg->readOnly = false;
+        return cfg->openStore();
+    }();
+
+    auto drvHash = Hash::parseExplicitFormatUnprefixed(
+        "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad", HashAlgorithm::SHA256, HashFormat::Base16);
+
+    auto outputName = "foo";
+
+    EXPECT_EQ(store->queryRealisation({drvHash, outputName}), nullptr);
+}
+
+} // namespace nix

src/libstore-tests/meson.build

@@ -61,6 +61,7 @@ sources = files(
   'derivation.cc',
   'derived-path.cc',
   'downstream-placeholder.cc',
+  'dummy-store.cc',
   'http-binary-cache-store.cc',
   'legacy-ssh-store.cc',
   'local-binary-cache-store.cc',