Addendum PR4988 - dead-lock in std.allocator.free_list unittest

[WalterWaldron]

https://issues.dlang.org/show_bug.cgi?id=16352

#4988 Fixed the cause, this allows the test to fail properly.

What was typically happening was

    private void decNodes() shared
    {
        assert(nodes);
        atomicOp!("-=")(nodes, 1);
    }

Would fail the assertion (because of data race.)
I don't know why scope(exit) tid.send(true); didn't allow the main thread to exit.

Anyway, instead I used ThreadGroup.joinAll to rethrow exceptions in the unit test thread.

[dlang-bot]

Fix	Bugzilla	Description
✓	16352	dead-lock in std.allocator.free_list unittest

[andralex]

thx