Add a function to create temporary directories for tests
Add a function to create temporary directories for tests

When we execute tests in the Maven context, the target/ directory is a
much better place to put stuff than /tmp/, especially when the tests are
run by Jenkins: the workspace including the target/ directories can be
accessed via Web, the /tmp/ directory cannot.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

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