tests: save to temporary directory

[ckamm]

There were two tests that wrote to the same file in tests/fixtures
which lead to the annoyance of manually having to revert the file
to avoid spurious commit changes. Probably the order of test
execution isn't stable.

With this change, the pytest 'tmpdir' fixture is used to provide a
unique temporary directory for each test.

[coveralls]

Pull Request Test Coverage Report for Build 360

• 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 57.994%

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Totals
Change from base Build 349:	0.0%
Covered Lines:	1440
Relevant Lines:	2483

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💛 - Coveralls