TestUtils: add a convenient function to create a temporary directory
TestUtils: add a convenient function to create a temporary directory

The TestUtils.createTemporaryDirectory() function used to require a
class to determine the context, but now we can offer a convenient
function that simply uses the calling for that.

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

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