Refactor test_runner.rs as a folder

[Centril]

This PR is a pure refactoring and should have zero semantic or public changes.

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I was originally planning on changing the signature of run from:

pub fn run<S, F>(&mut self, strategy: &S, test: F)
    -> Result<(), TestError<ValueFor<S>>>
where
    S : Strategy,
    F : Fn (&ValueFor<S>) -> TestCaseResult

to:

pub fn run<S, F>(&mut self, strategy: &S, test: F)
    -> Result<(), TestError<S::Value>> // <-- Note that the ValueTree is returned
where
    S : Strategy,
    F : Fn(ValueFor<S>) -> TestCaseResult // <-- Note the absence of & here.

but right now it didn't seem worth it to make that change. However - I think we should do that change once GATs land (and try it out in nightly) since it should allow us to address #9 as well as giving users an owned value in their tests without having to clone test case values (and not all types implement clone..).

[AltSysrq]

Thanks for this, it's increasingly felt necessary but I never got around to it.

FTR, your PR description contains the text fix #9, which github interprets to mean that merging this PR will close that issue; I'm going to take the liberty to edit that to something else to avoid close/reopen noise.

[Centril]

Oh thanks, I forgot about that =)