disable one more test for C++

tests/objects/tobjcov.nim

@@ -1,8 +1,12 @@
 discard """
 action: compile
+target: "c"
 """
 
 # Covariance is not type safe:
+# Note: `nim cpp` makes it a compile error (after codegen), even with:
+# `var f = cast[proc (x: var TA) {.nimcall.}](cast[pointer](bp))`, which
+# currently removes all the `cast` in cgen'd code, hence the compile error.
 
 type
   TA = object of RootObj