[SYCL] fix for flaky ~event failure in Win unit tests

[cperkinsintel]

Windows shutdown/teardown doesn't have the exact same timing as on Linux, and even more in the the unit tests which are statically compiled ( so no DllMain() call, which means no space between the calls to shutdown_early() and shutdown_late() ). Here we remove a race between the platform deletion in the mock and the win shutdown. This fixes a flaky failure we are seeing in a couple of the unit tests.

[uditagarwal97]

Windows shutdown/teardown doesn't have the exact same timing as on Linux, and even more in the the unit tests which are statically compiled ( so no DllMain() call, which means no space between the calls to shutdown_early() and shutdown_late() ). Here we remove a race between the platform deletion in the mock and the win shutdown.

Can you please elaborate more on how the race happens? Like rough thread interleavings?
I'm trying to think of an alternative approach, somewhat along the lines of: can we use mutexes to enforce an ordering between shutdown_late and shutdown_early, on Windows? To avoid any overhead, this mutex based shutdown ordering can be put under a special macro, that we define only when building unittests.