Reserve the vectors in ConcurrentHashMapTest (#998)
Reserve the vectors in ConcurrentHashMapTest (#998)

Summary:
Even though this is a test, we are still reallocating a vector (resizing until re-sized/reallocated to 32), which in theory calls for allocation 5 times. But we can save this by just reserving the vector pre-hand (we know the size).
Pull Request resolved: https://github.com/facebook/folly/pull/998

Reviewed By: magedm

Differential Revision: D13834292

Pulled By: yfeldblum

fbshipit-source-id: 6b0d16ef0200184d10ba292a0b1d8d3afdadf56d

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly