Consider filtering environments at the configuration level as we do with system partitions

[vkarak]

The reason we treat differently environments and partitions (aka systems) is that systems define the basic configuration scopes (not to be confused with the scopes requested in #1725). For example, an environment or any configuration parameter can be defined differently for different systems/partitions (see target_systems). This is not true for environments. However, at the test level, partitions and environments are treated equally. This needs a bit more thinking on how to treat environments properly, so I will simply add a FIXME note on this.

Originally posted by @vkarak in https://github.com/eth-cscs/reframe/pull/2166#discussion_r737872420