[IMT] Test thread safety of task arena and TThreadExecutor.
[IMT] Test thread safety of task arena and TThreadExecutor.

Create task arena pointers and TThreadExecutor instances in parallel.
This should prove that the task arena and ThreadExecutor are thread
safe, which was questioned in ROOT-10762.
{Dis|En}ableIMT might still be an issue, because it's a function-local
static without further protection.

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