tests: add prefixes to the bare mkdtemp calls

The first step to debug a thing is to know what created the thing in
the first place. Add some prefixes so random tmpdir's have something
grep in the code.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201117173635.29101-3-alex.bennee@linaro.org>