Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libstore-tests/local-overlay-store.cc

@@ -0,0 +1,34 @@
+// FIXME: Odd failures for templates that are causing the PR to break
+// for now with discussion with @Ericson2314 to comment out.
+#if 0
+#  include <gtest/gtest.h>
+
+#  include "local-overlay-store.hh"
+
+namespace nix {
+
+TEST(LocalOverlayStore, constructConfig_rootQueryParam)
+{
+    LocalOverlayStoreConfig config{
+        "local-overlay",
+        "",
+        {
+            {
+                "root",
+                "/foo/bar",
+            },
+        },
+    };
+
+    EXPECT_EQ(config.rootDir.get(), std::optional{"/foo/bar"});
+}
+
+TEST(LocalOverlayStore, constructConfig_rootPath)
+{
+    LocalOverlayStoreConfig config{"local-overlay", "/foo/bar", {}};
+
+    EXPECT_EQ(config.rootDir.get(), std::optional{"/foo/bar"});
+}
+
+} // namespace nix
+#endif