avocado.plugins: Improve the output of the 'plugins' subcommand

Order the available plugins and use colors to better
organize the information displayed:

$ scripts/avocado plugins
Plugins loaded:
    datadir - Implements the avocado 'datadir' subcommand (Enabled)
    json - JSON output (Enabled)
    journal - Test journal (Enabled)
    multiplexer - Implements the avocado 'multiplex' subcommand (Enabled)
    plugins_list - Implements the avocado 'plugins' subcommand (Enabled)
    run_vm - Run tests on a Virtual Machine (Enabled)
    sysinfo - Collect system information (Enabled)
    test_lister - Implements the avocado 'list' subcommand (Enabled)
    test_runner - Implements the avocado 'run' subcommand (Enabled)
    xunit - xUnit output (Enabled)

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
selftests/functional: basic_tests: Add plugin list test

Verify if plugins are correctly listed and none are
disabled.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests: Add datadir test

Create a test that uses files from the data dir, as
a functional test, so we notice when that functionality
is broken.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
selftests/functional: Change synctest with datadir

synctest fails on the CI VM, let's use the simpler,
bogus datadir test to do that functional verification.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>