Add printing of invariant_failedt in test failure messages

This means that generated error messages show what the particular
invariant failure actually generated was, instead of the catch framework
default of `{?}`.

Author: thomasspriggs

unit/testing-utils/invariant.cpp

@@ -27,3 +27,11 @@ std::string invariant_failure_containingt::describe() const
   return std::string{"invariant_failedt with `.what' containing - \""} +
          expected + "\"";
 }
+
+std::ostream &
+operator<<(std::ostream &out, const invariant_failedt &invariant_failed)
+{
+  out << "invariant_failedt where `.what()' is \"" << invariant_failed.what()
+      << "\"";
+  return out;
+}

unit/testing-utils/invariant.h

@@ -25,4 +25,8 @@ class invariant_failure_containingt
 invariant_failure_containingt
 invariant_failure_containing(std::string expected);
 
+/// Printing of `invariant_failedt` for test failure messages.
+std::ostream &
+operator<<(std::ostream &out, const invariant_failedt &invariant_failed);
+
 #endif // CPROVER_TESTING_UTILS_INVARIANT_H