avocado: Display error message and usage on empty 'run' subcommand

Instead of displaying a not very informative 'too few arguments'
message, display the original error message that mentions about
test IDs *and* the run help output. Adjustments to the original
functional test were made accordingly.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/plugins/runner.py

@@ -96,11 +96,11 @@ class TestRunner(plugin.Plugin):
             help='Run a list of test modules or dropin tests (space separated)')
 
         parser.add_argument('url', type=str, default=None,
-                            help=('Test module names or paths to dropin tests '
-                                  '(space separated)'))
+                            help=('List of test IDs (aliases or paths)'),
+                            nargs='?')
 
         parser.add_argument('-z', '--archive', action='store_true', default=False,
-                            help='Archive (ZIP) files generated by tests.')
+                            help='Archive (ZIP) files generated by tests')
 
         parser.add_argument('-m', '--multiplex-file', type=str, default=None,
                             help=('Path to an avocado multiplex '
@@ -108,7 +108,7 @@ class TestRunner(plugin.Plugin):
                             nargs='?')
 
         parser.add_argument('--keep-tmp-files', action='store_true', default=False,
-                            help='Keep temporary files generated by tests.')
+                            help='Keep temporary files generated by tests')
 
         parser.add_argument('--unique-id', type=str, default=None,
                             help=('Unique Job id. Used by a server when job '
@@ -116,6 +116,7 @@ class TestRunner(plugin.Plugin):
                                   'different test machine'))
 
         parser.set_defaults(func=self.run_tests)
+        self.parser = parser
         self.configured = True
 
     def run_tests(self, args):
@@ -125,7 +126,10 @@ class TestRunner(plugin.Plugin):
         :param args: Command line args received from the run subparser.
         """
         job_instance = job.Job(args)
-        return job_instance.run()
+        rc = job_instance.run()
+        if args.url is None:
+            self.parser.print_help()
+        return rc
 
 
 class SystemInformation(plugin.Plugin):

selftests/all/functional/avocado/basic_tests.py

@@ -168,9 +168,11 @@ class RunnerOperationTest(unittest.TestCase):
         cmd_line = './scripts/avocado run'
         result = process.run(cmd_line, ignore_status=True)
         expected_rc = 2
-        expected_output = 'avocado run: error: too few arguments'
+        expected_output = 'Empty test ID. A test path or alias must be provided'
+        expected_output_2 = 'usage:'
         self.assertEqual(result.exit_status, expected_rc)
         self.assertIn(expected_output, result.stderr)
+        self.assertIn(expected_output_2, result.stdout)
 
     def test_not_found(self):
         os.chdir(basedir)