test-runner.py: change shebang to python3

In commit 6e72a5b9b61066146deafda39ab8158c559f5f15 python scripts which
work with python2 and python3 changed the shebang from /usr/bin/python
to /usr/bin/python3. This gets adapted by the build-system on systems
which don't provide python3.
This commit changes test-runner.py to also use /usr/bin/python3,
enabling the change during buildtime and fixing a minor lintian issue
for those Debian packages, which depend on a specific python version
(python3/python2).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Closes #8803