Fix test_struct_union_c regression test

cbmc/havoc_slice/test_struct_union_c.desc regression test was using
test_struct_union_b.c instead of test_struct_union_c.c, that was never
used.

This commit fixes this by running the correct file.

Author: Enrico Steffinlongo

regression/cbmc/havoc_slice/test_struct_union_c.desc

@@ -1,5 +1,5 @@
 CORE no-new-smt
-test_struct_union_b.c
+test_struct_union_c.c
 
 ^EXIT=10$
 ^SIGNAL=0$