Fix timing issue in BufferedIOTest #198

Job	Run time
Homebrew packaging tests	6m 49s
C++ tests on macOS	3m 38s
C++ tests on Linux	1m 20s
Apache2 tests on Linux	2m 8s
Apache2 tests on macOS	2m 33s
Nginx dynamic module tests on Linux	1m 2s
Node.js tests on Linux	24s
Nginx tests on Linux	2m 17s
Nginx dynamic module tests on macOS	1m 38s
Ruby tests on Linux	1m 1s
Node.js tests on macOS	25s
Nginx tests on macOS	3m 51s
Ruby tests on macOS	2m 36s
Source packaging tests	31s
Passenger Standalone tests on Linux	4m 15s
Passenger Standalone tests on macOS	3m 50s
	38m 18s

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