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-binary-cache-store.cc

@@ -0,0 +1,14 @@
+#include <gtest/gtest.h>
+
+#include "local-binary-cache-store.hh"
+
+namespace nix {
+
+TEST(LocalBinaryCacheStore, constructConfig)
+{
+    LocalBinaryCacheStoreConfig config{"local", "/foo/bar/baz", {}};
+
+    EXPECT_EQ(config.binaryCacheDir, "/foo/bar/baz");
+}
+
+} // namespace nix