add the cpp target (#21164)

The issue is related to cpp codegen, the previous test doesn't test cpp backend, which will join into the megatest.

tests/cpp/t12946.nim

@@ -1,3 +1,7 @@
+discard """
+  targets: "c cpp"
+"""
+
 import std/atomics
 type Futex = distinct Atomic[int32]