This prevents an error when the avocado golang plugin is installed,
but "go" itself is not.

This still allows tests to be listed without "go" because:
 * it is not a requirement for that
 * there's value in listing tests even when your system is (still?)
   not completely setup to run them
 * Avocado hasn't promised symmetry between listing and guaranteeing
   that tests will be able to run
 * There may be situations in which we can *not* predict if tests
   that we can list can be executed
Signed-off-by: NCleber Rosa <crosa@redhat.com>