Skip lxc_template_unit_test.go on non-Linux platforms

It doesn't work without lxc-start.

Docker-DCO-1.1-Signed-off-by: Kato Kazuyoshi <kato.kazuyoshi@gmail.com> (github: kzys)
Merge pull request #6756 from kzys/lxc-linux

Skip lxc_template_unit_test.go on non-Linux platforms