Close the test dispatcher when completing

Currently, a test workflow like this will not ever run the code after the goroutine's sleep, nor the code in the defer:

func(ctx workflow.Context) error {
  workflow.Go(func(ctx workflow.Context) {
    defer func() { fmt.Println("in defer") }()
    workflow.Sleep(ctx, time.Hour) // wait longer than the workflow lives
    fmt.Println("after sleep")
  })
  workflow.Sleep(ctx, time.Minute) // just to make sure the goroutine starts
  return nil
}

The workflow will correctly end, but since the dispatcher was never closed, any not-yet-complete goroutines would never exit, and we'd leak goroutines.

Semantically this should be a safe change:

Any post-complete decisions would not be executed or recorded, and this retains that.
When panicking, anything that would be recorded in a defer will not be recorded, so no replay-state-aware user code should be affected. And any code that ignores replay state will now execute like it should, where before it would not.

So safe / correct code should be unaffected, leaks should be reduced, and latent mistakes should now cause errors. AFAICT - I'm not sure how complete our tests are here :)

There's some room for in-defer code to be semantically incorrect in tests without this fix, (e.g. testing custom logger/metric impls in defers), though I expect those to be very rare bordering on nonexistent. But for the most part I expect that people will not notice this change, they'll just have fewer goroutine leaks during tests (so e.g. https://github.com/uber-go/goleak users will be happy).

Prior to this fix, the added test fails with:

=== RUN   TestWorkflowUnitTest/Test_StaleGoroutinesAreShutDown
    internal_workflow_test.go:1210: 
            Error Trace:    internal_workflow_test.go:1210
            Error:          deferred func should have been called within 1 second
            Test:           TestWorkflowUnitTest/Test_StaleGoroutinesAreShutDown
    internal_workflow_test.go:1216: code after sleep correctly not executed

Now it passes with this, which also shows it's not slowing tests down in any meaningful way:

=== RUN   TestWorkflowUnitTest/Test_StaleGoroutinesAreShutDown
    internal_workflow_test.go:1210: deferred callback executed after 9.177µs
    internal_workflow_test.go:1217: code after sleep correctly not executed

Files with Coverage Reduction	New Missed Lines	%
internal/internal_task_pollers.go	8	80.52%
<!--	Total:	8		-->

Totals
Change from base Build 6e7ee6fa-6a5e-4c8b-b873-e06b32984d79:	-0.05%
Covered Lines:	12067
Relevant Lines:	16978

uber-go / cadence-client

Close the test dispatcher when completing #1117

Pull Request Test Coverage Report for Build 7f7a0549-3eae-484c-ac63-84b97b1d7cb4

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