add a simple ffi test using structs

tests-pl/ffi_struct.pl

@@ -0,0 +1,19 @@
+:- use_module(library(os)).
+:- use_module(library(ffi)).
+
+test :- 
+    read(Body),
+    term_variables(Body, [LIB]),
+    Body,
+    foreign_struct(pg, [uint8, uint16, uint32, uint64, uint8, f32, f64]),
+    use_foreign_module(LIB, ['construct'([uint8, uint16, uint32, uint64, uint8, f32, f64], pg), 'modify'([pg], pg)]),
+    ffi:'construct'(8, 12, 46, 40, 127, 1.368, -4.587, PG),
+    write(("PG"-PG)), nl,
+    ffi:'modify'(PG, [pg, A, B, C, D, A2, E, F]),
+    % skipping E & F for now as the result changes between runs for some reason
+    write(("PG2"-[pg, A, B, C, D, A2, "skip", "skip"])), nl,
+    % avoide singelton warning
+    E = _,
+    F = _.
+
+:- initialization(test).