Support ingredients, affecting order of tests

[Bodigrim]

Currently there is no simple way to write a tasty ingredient (ingredient transformer, to be precise), which affects order of test execution. Two common applications are:

• Randomize order of tests to detect implicit dependencies between them.
• Execute the longest running tests first, maximizing multithreaded CPU utilization.

I do not propose to implement such mechanisms in tasty itself, we should keep the core minimalistic. The missing part, which inhibits third parties from providing such functionality, is to provide a way to configure the order of [TestAction] in launchTestTree:

tasty/core/Test/Tasty/Run.hs

Lines 605 to 610 in f73ce42

	launchTestTree opts tree k0 = do
	(testActions, fins) <- createTestActions opts tree
	let NumThreads numTheads = lookupOption opts
	(t,k1) <- timed $ do
	abortTests <- runInParallel numTheads (testAction <$> testActions)
	(do let smap = IntMap.fromDistinctAscList $ zip [0..] (testStatus <$> testActions)

One could extend launchTestTree with an argument (OptionSet -> [TestAction] -> [TestAction]), but that would require us to expose data TestAction, which I deem undesirable:

tasty/core/Test/Tasty/Run.hs

Lines 304 to 313 in f73ce42

	data TestAction act = TestAction
	{ testAction :: act
	-- ^ Some action, typically 'UnresolvedAction', 'ResolvedAction', or 'Action'.
	, testPath :: Path
	-- ^ Path pointing to this action (a series of group names + a test name)
	, testDeps :: Seq Dependency
	-- ^ Dependencies introduced by AWK-like patterns
	, testStatus :: TVar Status
	-- ^ Status var that can be used to monitor test progress
	}

Instead I suggest adding an argument of type (OptionSet -> (a -> Path) -> [a] -> [a]: "if you tell me how to extract Path from a, I'll be able to reorder [a]".

How does it sound?