Make format unit test do something

6b88028 removed the existing test (for good reasons) but didn't
replace it with any other.

Author: tautschnig

unit/util/format.cpp

@@ -7,8 +7,14 @@
 \*******************************************************************/
 
 #include <util/bitvector_types.h>
-#include <util/format.h>
 #include <util/format_expr.h>
-#include <util/std_code.h>
+#include <util/std_expr.h>
 
 #include <testing-utils/use_catch.h>
+
+TEST_CASE("Format an expression.", "[core][util][format]")
+{
+  const signedbv_typet int_type{32};
+  plus_exprt plus{symbol_exprt{"foo", int_type}, symbol_exprt{"bar", int_type}};
+  REQUIRE(format_to_string(plus) == "foo + bar");
+}