tests: clean out more temporary files

What remains is a bunch of anonymous untraceable /tmp/tmp.XXXXXXXXXX
files and bak.root.receive.staff1.3835 from an error branch, testdir.1,
testdir.3, and testroot454470 (with children) in testroot

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259

contrib/pyzfs/libzfs_core/test/test_libzfs_core.py

@@ -1903,6 +1903,8 @@ class ZFSTest(unittest.TestCase):
             with self.assertRaises(lzc_exc.StreamIOError) as ctx:
                 lzc.lzc_send(snap, None, fd)
             os.close(fd)
+            os.unlink(output.name)
+
         self.assertEqual(ctx.exception.errno, errno.EBADF)
 
     def test_recv_full(self):