Add catch framework matcher for invariant_failuret testing

This utility supports writing more succinct requirements on
`invariant_failuret` being thrown and the reason for which it was
thrown.

Author: thomasspriggs

unit/testing-utils/Makefile

@@ -1,6 +1,7 @@
 SRC = \
   call_graph_test_utils.cpp \
   free_form_cmdline.cpp \
+  invariant.cpp \
   message.cpp \
   require_expr.cpp \
   require_symbol.cpp \

unit/testing-utils/invariant.cpp

@@ -0,0 +1,29 @@
+/// Author: Diffblue Ltd.
+
+#include "invariant.h"
+
+#include <utility>
+
+invariant_failure_containingt invariant_failure_containing(std::string expected)
+{
+  return invariant_failure_containingt{std::move(expected)};
+}
+
+invariant_failure_containingt::invariant_failure_containingt(
+  std::string expected)
+  : expected{std::move(expected)}
+{
+}
+
+bool invariant_failure_containingt::match(
+  const invariant_failedt &exception) const
+{
+  const std::string what = exception.what();
+  return what.find(expected) != std::string::npos;
+}
+
+std::string invariant_failure_containingt::describe() const
+{
+  return std::string{"invariant_failedt with `.what' containing - \""} +
+         expected + "\"";
+}

unit/testing-utils/invariant.h

@@ -0,0 +1,28 @@
+/// Author: Diffblue Ltd.
+
+#ifndef CPROVER_TESTING_UTILS_INVARIANT_H
+#define CPROVER_TESTING_UTILS_INVARIANT_H
+
+#include <testing-utils/use_catch.h>
+#include <util/invariant.h>
+
+#include <string>
+
+class invariant_failure_containingt
+  : public Catch::MatcherBase<invariant_failedt>
+{
+public:
+  explicit invariant_failure_containingt(std::string expected);
+  bool match(const invariant_failedt &exception) const override;
+  std::string describe() const override;
+
+private:
+  std::string expected;
+};
+
+/// Returns a matcher which matches an invariant_failedt exception, where the
+/// `.what()` returns a string containing \p expected.
+invariant_failure_containingt
+invariant_failure_containing(std::string expected);
+
+#endif // CPROVER_TESTING_UTILS_INVARIANT_H