[nrunner] improve avocado --verbose list

[willianrampazzo]

Is your feature request related to a problem? Please describe. When running avocado --verbose list, it still shows some outputs from the legacy runner

Describe the solution you'd like Remove outputs that are not meaningful to the --list command on nrunner.

Describe alternatives you've considered Ignore the ugly outputs :D

Additional information Steps to reproduce:

$ avocado --verbose list examples/gdb-prerun-scripts/
Type Test Tag(s)

Resolver             Reference                                Info
avocado-instrumented examples/gdb-prerun-scripts/README       File "examples/gdb-prerun-scripts/README" does not end with ".py"
exec-test            examples/gdb-prerun-scripts/README       File "examples/gdb-prerun-scripts/README" does not exist or is not executable
golang               examples/gdb-prerun-scripts/README
python-unittest      examples/gdb-prerun-scripts/README       File "examples/gdb-prerun-scripts/README" does not end with ".py"
robot                examples/gdb-prerun-scripts/README       File "examples/gdb-prerun-scripts/README" does not end with ".robot"
tap                  examples/gdb-prerun-scripts/README       File "examples/gdb-prerun-scripts/README" does not exist or is not executable
avocado-instrumented examples/gdb-prerun-scripts/pass-sigusr1 File "examples/gdb-prerun-scripts/pass-sigusr1" does not end with ".py"
exec-test            examples/gdb-prerun-scripts/pass-sigusr1 File "examples/gdb-prerun-scripts/pass-sigusr1" does not exist or is not executable
golang               examples/gdb-prerun-scripts/pass-sigusr1
python-unittest      examples/gdb-prerun-scripts/pass-sigusr1 File "examples/gdb-prerun-scripts/pass-sigusr1" does not end with ".py"
robot                examples/gdb-prerun-scripts/pass-sigusr1 File "examples/gdb-prerun-scripts/pass-sigusr1" does not end with ".robot"
tap                  examples/gdb-prerun-scripts/pass-sigusr1 File "examples/gdb-prerun-scripts/pass-sigusr1" does not exist or is not executable

TEST TYPES SUMMARY
==================

The header (Type Test Tag(s)) might be removed and the footer (TEST TYPES SUMMARY) might show meaningful information.