libexpr-tests: Add unit tests for broken readDir /. for pure eval

A very unfortunate interaction of current filtering with pure eval is
that the following actually leads to `lib.a = {}`. This just adds a unit
test for this broken behavior. This is really good to be done as a unit test
via the in-memory store.

{
  outputs =
    { ... }:
    {
      lib.a = builtins.readDir /.;
    };
}

src/libexpr-test-support/include/nix/expr/tests/libexpr.hh

@@ -26,11 +26,20 @@ public:
     }
 
 protected:
-    LibExprTest()
+    LibExprTest(ref<Store> store, auto && makeEvalSettings)
         : LibStoreTest()
+        , evalSettings(makeEvalSettings(readOnlyMode))
         , state({}, store, fetchSettings, evalSettings, nullptr)
     {
-        evalSettings.nixPath = {};
+    }
+
+    LibExprTest()
+        : LibExprTest(openStore("dummy://"), [](bool & readOnlyMode) {
+            EvalSettings settings{readOnlyMode};
+            settings.nixPath = {};
+            return settings;
+        })
+    {
     }
 
     Value eval(std::string input, bool forceValue = true)