fix test thread indexing

Author: IanButterworth

test/threads_exec.jl

@@ -525,7 +525,7 @@ function test_thread_cfunction()
         fi = fs[i]
         cfi = cf(fi)
         GC.@preserve cfi begin
-            ok[threadid()] += (cfi === cfs[i])
+            ok[threadpoolsize(:interactive) + threadid()] += (cfi === cfs[i])
         end
     end
     @test sum(ok) == 10000