List should exclude tests with empty PrgEnvs

[victorusu]

The -l cli flag relies on the self.valid_systems variable defined in the test.

One can set self.valid_prog_environs = []. This way, the test is listed in the prompt, but in fact, does not run on the system. It is skipped.

This is the correct behavior as stated in the documentation.

However, it does not reflect the reality of the actual list of tests that run on the system.