Skip memory_get_usage test if smartalloc is off

Summary: memory_get_usage(true) gives you different results depending on
whether the smart allocator is in use or not, so this test fails if you're
running an ASAN build.  This test doesn't feel that valuable in that scenario,
so just disable it.

Reviewed By: @edwinsmith

Differential Revision: D1749906

Signature: t1:1749906:1419004931:4ed4f1d1bef2f402620bb58bafc9e3b735307022