[P4Testgen] Validate options as early as possible

[fruffy]

With #4275 we added the possibility to validate the P4Testgen options in an explicit, second step. We should expand this to also validate options on a per-target basis, e.g., for v1model on BMv2, or pna on DPDK.

This way we can give feedback early when a test back end is not supported for a particular target, for example. This is currently not possible and a user only gets feedback after compiling for possibly several seconds.