libstore: Make sure that initNix has been called

Prevent bugs like https://github.com/cachix/cachix/pull/477

src/libexpr/tests/libexprtests.hh

@@ -12,6 +12,7 @@ namespace nix {
     class LibExprTest : public ::testing::Test {
         public:
             static void SetUpTestSuite() {
+                initLibStore();
                 initGC();
             }