Make each integration test independent

[shinnn]

The existing test results are too fragile to add new specs;

1. The snapshot of the first test affects the second one, the second affects the third, and so on.
2. The snapshot requires to be updated whenever the coverage of c8 itself changes.

This patch makes each test independent, by setting different --temp-directory flag to every spawnSync().

Checklist

• npm test, tests passing
• npm run test:snap (to update the snapshot)

[coveralls]

Pull Request Test Coverage Report for Build 161

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.852%

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Totals
Change from base Build 153:	0.0%
Covered Lines:	391
Relevant Lines:	401

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💛 - Coveralls