fix pool test for process generation incrementing #256

Job	Run time
Apache2 tests on Linux	2m 9s
C++ tests on Linux	1m 23s
Apache2 tests on macOS	3m 7s
C++ tests on macOS	4m 6s
Nginx tests on Linux	2m 11s
Nginx tests on macOS	2m 36s
Nginx dynamic module tests on Linux	1m 2s
Node.js tests on Linux	22s
Node.js tests on macOS	31s
Nginx dynamic module tests on macOS	1m 8s
Passenger Standalone tests on Linux	4m 3s
Passenger Standalone tests on macOS	4m 41s
Homebrew packaging tests	7m 56s
Source packaging tests	29s
Ruby tests on Linux	58s
Ruby tests on macOS	1m 7s
	37m 49s

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