Test: Turn `params` into a property

This change should decrease the probability of users accidentally
overriding `Test.params` while keeping the usage the same. There is one
caveat and that is Avocado-vt, which modifies `self.params`, which is
not that simple anymore. I have a fix for it, but the problem is it is
not backward compatible and it can only be changed to
forward-compatible. We should consider whether the benefit of having
this "a bit" protected is greater than backward incompatible change with
Avocado-vt (and possibly other "nasty" user's tests)
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/test.py

@@ -225,9 +225,9 @@ class Test(unittest.TestCase):
             params = []
         elif isinstance(params, tuple):
             params, mux_path = params[0], params[1]
-        self.params = varianter.AvocadoParams(params, self.name,
-                                              mux_path,
-                                              self.default_params)
+        self.__params = varianter.AvocadoParams(params, self.name,
+                                                mux_path,
+                                                self.default_params)
         default_timeout = getattr(self, "timeout", None)
         self.timeout = self.params.get("timeout", default=default_timeout)
 
@@ -288,6 +288,13 @@ class Test(unittest.TestCase):
         """
         return self.__outputdir
 
+    @property
+    def params(self):
+        """
+        Parameters of this test (AvocadoParam instance)
+        """
+        return self.__params
+
     @property
     def basedir(self):
         """