occasional crashes with last test-parallel_reduce.R test

[paciorek]

In labelAbstractTypes handler ParallelReduce: too many levels of class hierarchy in reduction operator o$y$plus
 This occurred for: parallel_reduce(operator = o$y$plus, object = x, nThreads = 0)
 This was part of the call:  y <- parallel_reduce(operator = o$y$plus, object = x, nThreads = 0)
This occurred while: handling labelAbstractTypes for parallel_reduce.
[ FAIL 0 | WARN 0 | SKIP 0 | PASS 45 ]*** longjmp causes uninitialized stack frame ***: terminated
Aborted

AI indicates this has to do with using TBB with R being single threaded. As far as I can tell things have always been fine outside the testing context, so just noting this for now.