[tests] Add a test for empty unions in MSVC machdep

tests/syntax/empty_union.i

 /* run.config*
    STDOPT: +"-machdep gcc_x86_32 -print -ocode @PTEST_NAME@_reparse.c -then @PTEST_NAME@_reparse.c -ocode=''"
+   STDOPT: +"-machdep msvc_x86_64 -print -ocode @PTEST_NAME@_reparse.c -then @PTEST_NAME@_reparse.c -ocode=''"
  EXIT: 1
    STDOPT:
  */

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.
+[kernel] Parsing empty_union_reparse.c (with preprocessing)
+/* Generated by Frama-C */
+union empty {
+   
+};
+union empty eu = {};
+

tests/syntax/oracle/empty_union.2.res.oracle

+[kernel] Parsing empty_union.i (no preprocessing)
+[kernel] empty_union.i:9: 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:9: 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
+  7     
+  8     // based on GCC's 'torture' test suite
+  9     union empty {} eu = {};
+                       ^^
+[kernel] Frama-C aborted: invalid user input.