Glib Plugin: don't attempt to run test reference if not an executable

This prevents the plugin from trying to run references such as
Avocado-VT tests or any other that is not backed by an executable
file.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/glib/avocado_glib/__init__.py

@@ -16,8 +16,10 @@
 Plugin to run GLib Test Framework tests in Avocado
 """
 
+import os
 import re
 
+from avocado.utils import path
 from avocado.utils import process
 
 from avocado.core import loader
@@ -82,6 +84,9 @@ class GLibLoader(loader.TestLoader):
         if ':' in reference:
             reference, _subtests_filter = reference.split(':', 1)
             subtests_filter = re.compile(_subtests_filter)
+
+        if (os.path.isfile(reference) and
+                path.PathInspector(reference).has_exec_permission()):
             try:
                 cmd = '%s -l' % (reference)
                 result = process.run(cmd)