tests: make `mktemp` command call work with Chimera Linux's `mktemp`

On Chimera Linux, which uses FreeBSD's userland tools, the original call fails with the following error:

mktemp: mkstemp failed on /tmp/libfprint-XXXXXX.hwdb: Invalid argument

Moving the X's to the end of the template passed to mktemp fixes the error, and also works with GNU's mktemp.