Add support for sharding tests with existing test times in typ

[nedn]

If user pass in a file that contains data about test's expected runtime, typ should be able to use it to shard the tests smarter.

[kenrussell]

Per https://bugs.chromium.org/p/chromium/issues/detail?id=636153#c3 I wonder whether this logic really has to live in typ. If the caller of typ can specify a subset of tests to run, that's sufficient to be able to implement this algorithm.