[GWP-ASan] Fix flaky test on Fuchsia

The LateInit test might be reusing some already initialized thread
specific data if run within the main thread. This means that there
is a chance that the current value will not be enough for the 100
iterations, hence the test flaking.

Fix this by making the test run in its own thread.

Differential Revision: https://reviews.llvm.org/D92415