Fix sync_once_cell_does_not_leak_partially_constructed_boxes
Fix sync_once_cell_does_not_leak_partially_constructed_boxes

Spinning multiple threads in this test causes a deadlock in
SGX where thread scheduling is not preemptive.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust