Fix tests on FS without inode count (#146)

On btrfs inode count is always 0, so test would fail.

Author: thrasibule

tests/test_sftp.py

@@ -256,7 +256,7 @@ def test_statvfs(self):
         sftp = self.session.sftp_init()
         vfs = sftp.statvfs('.')
         self.assertTrue(vfs is not None)
-        self.assertTrue(vfs.f_files > 0)
+        self.assertTrue(vfs.f_files >= 0)
         self.assertTrue(vfs.f_bsize > 0)
         self.assertTrue(vfs.f_namemax > 0)
 
@@ -272,7 +272,7 @@ def test_fstatvfs(self):
             with sftp.open(remote_filename, 0, 0) as fh:
                 vfs = fh.fstatvfs()
                 self.assertTrue(vfs is not None)
-                self.assertTrue(vfs.f_files > 0)
+                self.assertTrue(vfs.f_files >= 0)
                 self.assertTrue(vfs.f_bsize > 0)
                 self.assertTrue(vfs.f_namemax > 0)
         except Exception: