add ffi tests using f64

tests-pl/ffi_f64_nan.pl

@@ -0,0 +1,10 @@
+:- use_module(library(os)).
+:- use_module(library(ffi)).
+
+test :- 
+    getenv("ffi_f64_nan_LIB", LIB),    
+    use_foreign_module(LIB, ['ffi_f64_nan'([], f64)]),
+    ffi:'ffi_f64_nan'(N),
+    _ is round(N).
+
+:- initialization(test).