Tests: fix detecting Python on systems with only "python3" command #272

Job	Run time
C++ tests on Linux	2m 7s
Apache2 tests on Linux	2m 45s
Apache2 tests on macOS	2m 40s
C++ tests on macOS	4m 31s
Homebrew packaging tests	7m 41s
Nginx tests on Linux	2m 55s
Nginx dynamic module tests on Linux	1m 32s
Nginx tests on macOS	2m 32s
Node.js tests on Linux	31s
Nginx dynamic module tests on macOS	1m 25s
Ruby tests on Linux	1m 26s
Node.js tests on macOS	27s
Ruby tests on macOS	1m 12s
Source packaging tests	30s
Passenger Standalone tests on Linux	3m 53s
Passenger Standalone tests on macOS	4m 42s
	40m 49s

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