Add unit tests for LimitedConcurrencyLevelTaskScheduler, #1110

[paulirwin]

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Adds unit tests for LimitedConcurrencyLevelTaskScheduler.

Fixes #1110

Description

This adds some Apache Harmony tests for ThreadPoolExecutor and adapts them for testing LimitedConcurrencyLevelTaskScheduler (used in Lucene.NET in place of ThreadPoolExecutor).

This class, being a TaskScheduler, is only useful when used with i.e. a TaskFactory, so it does not have a very testable public API surface. So to exercise this class, some extension methods were added in the JSR166TestCase file to emulate a ThreadPoolExecutor's API surface. This keeps track of task state using a ConditionalWeakTable keyed on TaskScheduler, so that when the TaskScheduler is garbage collected, its state is cleaned up. The state is needed to track completed tasks, since LimitedConcurrencyLevelTaskScheduler removes/dequeues tasks when they are completed. This also implements the joinPool method in JSR166TestCase via waiting for all tracked tasks for the scheduler with a timeout.

This does not port all of the tests from Harmony, since most are irrelevant, but it ports enough to test executing tasks, concurrency level, the number of running/completed tasks, shutdown state, etc.

As noted in #1110, FEATURE_THREADPOOL_UNSAFEQUEUEWORKITEM was removed as all targets support this feature.