Test: turn basedir into a property

Since basedir is closely related to `filename` and `datadir`, let's
also make it a property for coherency.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/test.py

@@ -88,8 +88,6 @@ class Test(unittest.TestCase):
 
         self.job = job
 
-        self.basedir = os.path.dirname(self.filename)
-
         if self.datadir is None:
             self._expected_stdout_file = None
             self._expected_stderr_file = None
@@ -162,6 +160,16 @@ class Test(unittest.TestCase):
         self.time_elapsed = None
         unittest.TestCase.__init__(self, methodName=methodName)
 
+    @property
+    def basedir(self):
+        """
+        The directory where this test (when backed by a file) is located at
+        """
+        if self.filename is not None:
+            return os.path.dirname(self.filename)
+        else:
+            return None
+
     @property
     def datadir(self):
         """