Fix failing specs when test host uses base service provider

[jcharaoui]

Short description

In some build environments such as Debian's pbuilder, the host's /proc might be mounted in the build environment, but the systemctl executable is missing from the chroot, since it's not a strict build dependency.

In such environments, the base service provider will be selected, which doesn't support the :enable parameter, leading to such failures:

Service[yay](provider=base) does not implement: enable

Instead of forcing the host's service provider to report support for the enable parameter, simply skip the test when it isn't available.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)