TestHRegion and TestHRegionWithInMemoryFlush timing out

These two tests are marked LargeTests and thus have a timeout of 10 minutes, but it's not sufficient since they have like more than 100 testcases.
Ideal fix would be splitting the tests, but that'll be much more involved effort.
For now, increasing the timeout to 15min.

Current stats:
On apache infra:
TestHRegionWithInMemoryFlush 20.0% (15 / 75) 15 / 12 / 0
TestHRegion 9.3% (7 / 75) 7 / 5 / 0

On GCE infra:
TestHRegion 89.7% (87 / 97) 87 / 87 / 0
TestHRegionWithInMemoryFlush 71.1% (69 / 97) 69 / 69 / 0