avocado.utils.ssh: improve connection establishment check

Instead of relying only on the SSH status code, which can succeed
while still not leaving a usable master connection running, let's add
an explicit check.

Reference: https://github.com/avocado-framework/avocado/issues/2900Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/utils/ssh.py

@@ -77,7 +77,7 @@ class Session(object):
             if not master.exit_status == 0:
                 return False
             self._connection = master
-        return True
+        return self._check()
 
     def cmd(self, command):
         """