sync: run most tests which modify the host only in CI

This is especially important in case the user executes the tests as root
(no recommended but not prevented either). Permissions on paths like /
or /tmp which differ from those expected by the test could otherwise be
modified by the tests.

However, the end-to-end SSH tests which write /tmp/safcm-remote-$uid
(but no other paths) are still run so we get proper coverage of basic
features.