[#1856] Check current working folder for temporary files.


Problem description
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If a test was creating a temporary file/folder in working folder the testcase teardown did not complain about it.




Changes description
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Add a method to check temporary files in working folder and add it to default teardown.

Tests were added for this.

I have also made a few drive by fixed for AssertError unicode handlie... AssertionError does not support unicode.


How to try and test the changes
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reviewers @alibotean 

Check that changes make sense.