Use GCHandle + plain void instead of async void in xUnit RunTest

Replace async void with plain void for the ExecutionContext.Run callback.
The test body runs in an async Task method (runTestCore) whose returned
Task is pinned via GCHandle.Alloc — a root in the GC handle table that
is independent of the managed object graph. This is stronger than
GC.KeepAlive and avoids async void entirely.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericstj