Dispose a test's container when its case has run

[ipjohnson]

What

ModuleTestCase now implements ISelfExecutingXunitTestCase: it runs the case the way xUnit would have, through XunitRunnerHelper.RunXunitTestCase, and disposes every provider it built in a finally once the run returns.

Why

The provider went into the case's DisposalTracker, and xUnit disposes a test case only in InProcessFrontController.FindAndRun, after Run has returned for every test case in the assembly. So every container a run built stayed alive, with every singleton in it, until the run ended. A probe in Hardened.Framework that handed three [HardenedTest] providers to an [assembly: AssemblyFixture] found all three undisposed at the end of the run. The NUnit integration has always disposed in ModuleTestCommand's finally, and its IterationLifetimeTests holds it to that; nothing held xUnit to it.

Hardened is about to add test hosts that bind a loopback socket per test, and a listener per test held open until the last test is the leak that has to be ruled out first.

Notes

• Per case, which for every test but a data-driven one is per test. The rows of a data-driven test share the case and are released together when the last has run. Per row would need a case runner of this package's own overriding RunTest; not done here.
• ContainerLifetimeTests in integ-tests/SutProject.Tests mirrors the NUnit IterationLifetimeTests: two tests in one class, whichever runs second asserts the first's container was disposed. It fails on main (Assert.Contains() Failure: Item not found in collection: []) and passes here, on both target frameworks.
• Hardened.Framework's whole suite, 7054 tests, passes with this runner wired in through the sibling-checkout project reference.
• The xUnit public API snapshot gains the interface and the Run method.
• No CHANGELOG entry, following Seed the test's environment before its modules apply #54; the cut commit carries it. Hardened takes the bump, so a 1.3.1 cut is what it waits on.

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