test-execute: skip tests that are broken without unprivileged userns

test-execute: skip tests that are broken without unprivileged userns

With newer versions of AppArmor, unprivileged user namespace creation
may be restricted by default, in which case user manager instances will
not be able to apply PrivateUsers=yes (or the settings which require it).
Additionally, if a kernel has the kernel.unprivileged_userns_clone
sysctl patch, and that sysctl is 0, then unprivileged userns creation
will always fail.

If a test unit is going to be run in a user manager, and that unit
requires PrivateUsers=yes (explicitly or implicitly), then skip it if
we do not have user namespace privileges.