Use `path.join` when handling file paths

[cmfcmf]

@corinnaj I've extracted this change from #62 and also expanded it to the tests, as suggested.

[coveralls]

Pull Request Test Coverage Report for Build 97

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.094%

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Totals
Change from base Build 93:	0.0%
Covered Lines:	297
Relevant Lines:	301

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💛 - Coveralls

[cmfcmf]

There even is an eslint rule for this: eslint.org/docs/rules/no-path-concat, we could consider enabling that

Good idea, @corinnaj! I've activated the rule in 80397d8. @frederike-ramin the rule's docs also give some more information on why it makes sense to use path.join: https://eslint.org/docs/rules/no-path-concat