[FakeLowp] Reset workspace in test (#37799)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37799
Failure to do so will results on some workspace contention.
Test Plan: unittest
Reviewed By: amylittleyang
Differential Revision: D21390900
fbshipit-source-id: 9e837f0f7aae32230740604069308f35b73612b9
