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change `postinstall` to `pretest` script. #212

Job	Run time
Test on Linux Firefox & Chrome	20s
Test on Node.js (18.x)	22s
Lint	26s
Test on Windows Firefox, Chrome & Edge	1m 41s
Test on Node.js (20.x)	29s
Test on Node.js (21.x)	25s
	3m 43s

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