test: add tests for function equality behavior

Add tests for function equality covering both direct comparisons and
comparisons within composite types (lists and attribute sets).

Tests verify:
- Direct function comparisons always return false
- Value identity optimization in composite types allows identical
  functions to compare as equal when both references point to the
  same function value

tests/functional/lang/eval-okay-equal-function-attrset-distinct-similar.nix

@@ -0,0 +1,3 @@
+# Distinct but not identical functions in attribute set compare as unequal
+# See https://nix.dev/manual/nix/latest/language/operators#equality
+{ a = (x: x); } == { a = (x: x); }