Add test for shadow memory of __errno

We must be able to attach shadow memory to the
memory returned by __errno().

Author: peterschrammel

regression/cbmc-shadow-memory/errno1/main.c

@@ -0,0 +1,15 @@
+#include <assert.h>
+#include <errno.h>
+
+int main()
+{
+  __CPROVER_field_decl_local("field1", (_Bool)0);
+  __CPROVER_field_decl_global("field1", (_Bool)0);
+
+  int *error = __errno();
+
+  assert(__CPROVER_get_field(error, "field1") == 0);
+
+  __CPROVER_set_field(error, "field1", 1);
+  assert(__CPROVER_get_field(error, "field1") == 1);
+}

regression/cbmc-shadow-memory/errno1/test.desc

@@ -0,0 +1,8 @@
+FUTURE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+^VERIFICATION SUCCESSFUL$
+--
+^warning: ignoring