chore: ignore test files in npm package

[JKRhb]

This PR is a potential fix for the problem that tests files end up on npm. The change to the .npmignore file should prevent that in the future.

[coveralls]

Pull Request Test Coverage Report for Build 1427743455

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 87.444%

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Totals
Change from base Build 1427722373:	0.1%
Covered Lines:	1131
Relevant Lines:	1244

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💛 - Coveralls

[Apollon77]

BTW: I also had that error in a local test https://github.com/mcollina/node-coap/runs/4122788149?check_suite_focus=true#step:6:578

[Apollon77]

Restart tests most enough