TST: Fix dtype mismatch on 32bit in IntervalTree get_indexer test
xref https://github.com/pandas-dev/pandas/pull/23442#issuecomment-435340042

To address @jreback's comment in the xref:
> `.get_indexer` should be returning platform int - but it’s not ? (maybe just this case)

It looks like `IntervalTree.get_indexer` and any other similar `IntervalTree` methods (`get_loc`, `get_indexer_nonunique`, etc.) will always return int64 as the data comes from a `Int64Vector.to_array()`.  

If returning platform int is expected, I'm not sure that making this change now is the best use of time: I'm hitting these methods as part of the new `IntervalIndex` behavior specs, so probably best enforce platform int as part of that implementation.