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.

src/catch2/internal/catch_commandline.cpp

@@ -170,6 +170,9 @@ namespace Catch {
             | Opt( [&]( bool flag ) { config.showDurations = flag ? ShowDurations::Always : ShowDurations::Never; }, "yes|no" )
                 ["-d"]["--durations"]
                 ( "show test durations" )
+            | Opt( config.minDuration, "seconds" )
+                ["-D"]["--min-duration"]
+                ( "show test durations for tests taking at least the given number of seconds" )
             | Opt( loadTestNamesFromFile, "filename" )
                 ["-f"]["--input-file"]
                 ( "load test names to run from a file" )