Fix macOS winetest timeout in kernel32:virtual
Work around slow runtime of current FlushProcessWriteBuffers() implementation on macOS.
The total iteration count becomes:
- positive half: 32768 * 1 (old) = 4096 * 8 (new)
- negative half: 32768 * 1 (old) > 4096 * 2 (new)
Edited by Jinoh Kang