Add support for non-fork-join test executor

[asardaes]

Overview

Since there wasn't much feedback in #3108, I figured I'd open a PR with what I have to see if it helps. I reiterate some comments from the linked issue and add some more info.

The idea is to allow a different executor with a fixed thread pool to execute actual tests in parallel in order to avoid potential conflicts with user code if they also rely on the JVM's fork-join APIs. The default would remain as before. Users could then change the executor via configuration.

I wrote a couple of TODOs, but they're both related to my understanding of CONCURRENT execution mode semantics. The current implementation checks execution mode in both submit and forkConcurrentTasks + executeNonConcurrentTasks, but in both scenarios, if the task is not concurrent, it executes it in the current thread, and that doesn't know if some other thread in the pool has stolen work and is executing another test, so is it really guaranteeing that no other test is running at the same time?

I added a method ExclusiveTask#await and use it instead of join in joinConcurrentTasksInReverseOrderToEnableWorkStealing - if the task should execute in a ForkJoinWorkerThread, await delegates to join, otherwise it delegates to compute. If I don't do it like this, the tests execute fine and are even show as PASSED in the terminal, but after that everything stalls and all threads in the fork-join pool remain parked, and I cannot figure out why. I tried to reproduce this outside of a JUnit 5 session and I couldn't.

I added FixedThreadsParallelExecutionIntegrationTests as an almost identical copy of ParallelExecutionIntegrationTests but using the fixed thread pool for test execution, and I can see that there are a lot of failures. I spent some time looking through the logic to see if I could figure out how it's set up, but I couldn't make much progress, I would appreciate feedback if I'm understanding the tests incorrectly. I will point out something more about this in the comments below.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[asardaes]

The canRunTestsIsolatedFromEachOtherWithNestedCases test from FixedThreadsParallelExecutionIntegrationTests was stuck in a deadlock until I added these lines. I spent some time debugging and it seems to me that all of the org.junit.platform.engine.support.hierarchical.SingleLock instances created for the tasks are somehow locked by a fork-join thread before the tasks are scheduled in the fixed thread pool, but no matter how many breakpoints I added, I couldn't figure out where that's happening.