tests: move fixtures to dedicated dir

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I9d6ce6a264780f215b1b57d947b5264c6a6a6964

tests/fixtures/list_concat.nix

@@ -0,0 +1,15 @@
+# Test list concatenation operator ++
+let
+  list1 = [1 2 3];
+  list2 = [4 5 6];
+  empty = [];
+in {
+  # Basic concatenation
+  combined = list1 ++ list2;
+
+  # Concatenate with empty list
+  with_empty = list1 ++ empty;
+
+  # Nested concatenation
+  triple = [1] ++ [2] ++ [3];
+}