Add --min-duration option

A test runner already has a --durations option to print durations.
However, this isn't entirely satisfactory.

When there are many tests, this produces output spam which makes it hard
to find the test failure output.  Nevertheless, it is helpful to be
informed of tests which are unusually slow.

Therefore, introduce a new option --min-duration that causes all
durations above a certain threshold to be printed.  This allows slow
tests to be visible without mentioning every test.

tests/SelfTest/TimingTests/Sleep.tests.cpp

@@ -0,0 +1,23 @@
+/*
+ *  Copyright 2011 Two Blue Cubes Ltd. All rights reserved.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include <catch2/catch_test_macros.hpp>
+
+#include <chrono>
+#include <thread>
+
+TEST_CASE( "sleep_for_100ms", "[.min_duration_test][approvals]" )
+{
+  std::this_thread::sleep_for( std::chrono::milliseconds( 100 ) );
+  CHECK( true );
+}
+
+TEST_CASE( "sleep_for_200ms", "[.min_duration_test][approvals]" )
+{
+  std::this_thread::sleep_for( std::chrono::milliseconds( 200 ) );
+  CHECK( true );
+}