[tests] add test case for empty union with initializer

tests/syntax/empty_union.i

+/* run.config*
+   STDOPT: +"-machdep gcc_x86_32"
+ EXIT: 1
+   STDOPT:
+ */
+
+// based on GCC's 'torture' test suite
+union empty {} eu = {};

tests/syntax/oracle/empty_union.0.res.oracle

+[kernel] Parsing empty_union.i (no preprocessing)
+/* Generated by Frama-C */
+union empty {
+   
+};
+union empty eu = ;
+

tests/syntax/oracle/empty_union.1.res.oracle

+[kernel] Parsing empty_union.i (no preprocessing)
+[kernel] empty_union.i:8: User Error: 
+  empty unions only allowed for GCC/MSVC machdeps; see option -machdep or run 'frama-c -machdep help' for the list of available machdeps
+[kernel] empty_union.i:8: User Error: 
+  empty initializers only allowed for GCC/MSVC machdeps; see option -machdep or run 'frama-c -machdep help' for the list of available machdeps
+  6     
+  7     // based on GCC's 'torture' test suite
+  8     union empty {} eu = {};
+                       ^^
+[kernel] Frama-C aborted: invalid user input.