fix failing enable_finalizers test

(cherry picked from commit 43c7d02)

Author: JeffBezanson

test/misc.jl

@@ -136,7 +136,10 @@ for l in (Threads.SpinLock(), ReentrantLock())
     @test get_finalizers_inhibited() == 1
     GC.enable_finalizers(true)
     @test get_finalizers_inhibited() == 0
-    @test_warn "WARNING: GC finalizers already enabled on this thread." GC.enable_finalizers(true)
+    if ccall(:jl_is_debugbuild, Cint, ()) != 0
+        # Note this warning only exists in debug builds
+        @test_warn "WARNING: GC finalizers already enabled on this thread." GC.enable_finalizers(true)
+    end
 
     @test lock(l) === nothing
     @test try unlock(l) finally end === nothing