Avoid using withExtendedLifetime in unit tests

[calda]

Summary

This PR proposes a rule to avoid using withExtendedLifetime in unit tests.

Reasoning

We found internally that none of the 400+ uses of withExtendedLifetime in our test suite actually affect runtime behavior since a variable declared in a function scope is never deallocated before the end of that scope.

// WRONG
@Test
func `telescope tracks target`() {
  let observatory = Observatory()
  let telescope = observatory.veryLargeTelescope()

  telescope.track(.mars)
  #expect(telescope.isTracking)

  // Ensure the observatory isn't deallocated before we finish our observation
  withExtendedLifetime(observatory) { }
}

// RIGHT
@Test
func `telescope tracks target`() {
  let observatory = Observatory()
  let telescope = observatory.veryLargeTelescope()

  telescope.track(.mars)
  #expect(telescope.isTracking)
}

However, it is sometimes used to suppress an "unused variable" warning. This sort of usage is fine:

// ALSO RIGHT. Without `withExtendedLifetime`, `cancellable` would cause an "unused variable" warning.
@Test
func `telescope publishes target updates`() {
  let telescope = Telescope()
  var publishedTarget: Planet?
  let cancellable = telescope.targetPublisher.sink { publishedTarget = $0 }

  telescope.track(.mars)
  #expect(publishedTarget == .mars)

  withExtendedLifetime(cancellable) { }
}