Add instance IsTest (ContT () IO t)

[Bodigrim]

Following up on our discussion with @mrkline at Bodigrim/tasty-bench#69, how do we feel about adding instance IsTest t => IsTest (ContT () IO t)? Essentially it provides a way to run a test with a resource, which is

• more lightweight than withResource, which operates over TestTrees and is great to share a resource between tests but not that great if you want a single test,
• allows to deal with a more restrictive resource-providing API (imagine that a certain resource is provided only by withFoo combinator instead of having both openFoo and closeFoo exposed).

The new instance does not cost us anything in terms of dependencies. Any opinions?