Allow deferred variables in 'only_if' and 'not-if'.

[Paul-Ferrell]

It seems, you cannot use deferred variables (e.g. host_arch) in the only_if section of tests. In combination with the slurm scheduler, this is problematic. The only_if is evaluated on the frontend without deferred variables, so if you have e.g. an x86_64 frontend but aarch64 compute nodes, the tests are never built nor ran. In my case, I have a sys_var plugin for the vectorization support (SSE, AVX, AVX512, …) of the “host”. The frontend is an old Haswell system (so no AVX512) while the backend are IcelakeSP nodes (support for AVX512). How can we fix this?