tests: Fix a clang possible memory leak warning
clang was reporting a possible memory leak after mkdtemp() call, which
was a false positive, since mkdtemp() returns the same pointer provided
as the parameter, in case of success. This changes the code so that the
static analyser don't get confused.
Found by csbuild runner.
Signed-off-by: Anderson Toshiyuki Sasaki <email@example.com>
Reviewed-by: Andreas Schneider <firstname.lastname@example.org>