Work around slow runtime of current FlushProcessWriteBuffers() implementation on macOS.
The total iteration count becomes:
``` - positive half: 32768 * 1 (old) = 8192 * 4 (new) - negative half: 32768 * 1 (old) > 8192 * 1 (new) ```
-- v3: kernel32/tests: Shorten time for negative half of litmus test for FlushProcessWriteBuffers(). kernel32/tests: Run store_buffer_litmus_test() in a single-iteration loop.