Add regression test for smt2 union support with --trace

Author: thomasspriggs

regression/cbmc-incr-smt2/unions/padded.c

@@ -0,0 +1,14 @@
+#include <assert.h>
+#include <stdint.h>
+
+union my_uniont
+{
+  uint16_t a;
+  uint8_t b;
+};
+
+int main()
+{
+  union my_uniont my_union = {.b = 5};
+  assert(my_union.a == 5);
+}

regression/cbmc-incr-smt2/unions/padded.desc

@@ -0,0 +1,12 @@
+CORE
+padded.c
+--trace
+Passing problem to incremental SMT2 solving
+\[main\.assertion\.1\] line 13 assertion my_union\.a \=\= 5\: FAILURE
+my_union\=\{ \.a\=\d+ \} \(\d{8} 00000101\)
+^EXIT=10$
+^SIGNAL=0$
+--
+--
+Test that we support union values and traces for a example where the trace
+should include non deterministic padding.