about using test files in our unit tests

Hey,

so a quick note about using test files in our unit tests. As I had to learn
the hard way we can not reuse test files from one test in another unit
test. Especially on windows this is going to cause random build failures
when two tests try to access the same file in parallel.

I also hope that I fixed the source of many random test failures on windows
that seem to be related to reused tmp files. If you are still seeing random
test failures on the jenkins windows builds please drop me a note.

Regards,
Markus