docs: Add note regarding public properties

This should help beginners to identify the issue when accidentally
overriding test's public variables. Also add a unittest to support this
note.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

docs/source/WritingTests.rst

@@ -58,6 +58,10 @@ Note that the test class provides you with a number of convenience attributes:
   you can find that more information at :doc:`Mux`.
 * And many more (see :mod:`avocado.core.test.Test`)
 
+To minimize the accidental clashes we define the public ones as properties
+so if you see something like ``AttributeError: can't set attribute`` double
+you are not overriding these.
+
 Test statuses
 =============
 

selftests/unit/test_test.py

@@ -98,6 +98,11 @@ class TestClassTestUnit(unittest.TestCase):
         self.assertRaises(exceptions.TestSetupFail, self.DummyTest, "test",
                           test.TestName(1, "name"), base_logdir=self.tmpdir)
 
+    def testTryOverrideTestVariable(self):
+        test = self.DummyTest(base_logdir=self.tmpdir)
+        self.assertRaises(AttributeError, setattr, test, "name", "whatever")
+        self.assertRaises(AttributeError, setattr, test, "status", "whatever")
+
 
 class TestClassTest(unittest.TestCase):