Update MallctlHelperTest with jemalloc 5 APIs.
Update MallctlHelperTest with jemalloc 5 APIs.

Summary: Update decay time API to be jemalloc 4 & 5 compatible.

Reviewed By: yfeldblum

Differential Revision: D5188574

fbshipit-source-id: 8e3c26c8b6431efdd0ec7ff492a065bddb816b4b

rklabs/folly,rklabs/folly,Orvid/folly,Orvid/folly,Orvid/folly,facebook/folly,facebook/folly,facebook/folly,Orvid/folly,rklabs/folly,Orvid/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly