add ffi tests using f64

tests-pl/ffi_f64_minus_zero.pl

@@ -0,0 +1,16 @@
+:- use_module(library(os)).
+:- use_module(library(ffi)).
+
+test :- 
+    getenv("ffi_f64_minus_zero_LIB", LIB),    
+    use_foreign_module(LIB, ['ffi_f64_minus_zero'([], f64), 'signum'([f64], f64)]),
+    ffi:'ffi_f64_minus_zero'(N),
+    A is max(0.0, N),
+    B is max(N, 0.0),
+    ffi:'signum'(A, SA),
+    ffi:'signum'(B, SB),
+    write((SA, SB)),
+    -1.0 is SA, % incorrect, based on https://www.swi-prolog.org/pldoc/man?function=max/2 -0.0 is less than 0.0 so A and B should be 0.0 for which signum should be 1
+    1.0 is SB.
+
+:- initialization(test).