padding system test is timing sensitive

Job #487976 failed for 299fdfc7:

"rndc stats" can take a while to complete. checkpoint the stats file before each call. Wait a while for the stats to be written.

S:padding:Wed Dec 18 00:37:32 UTC 2019
T:padding:1:A
A:padding:System test padding
I:padding:PORTRANGE:10400 - 10499
I:padding:checking that dig handles padding (0)
I:padding:checking that dig added padding (1)
I:padding:checking that padding is added for TCP responses (2)
I:padding:checking that padding is added to valid cookie responses (3)
I:padding:checking that padding must be requested (TCP) (4)
I:padding:checking that padding must be requested (valid cookie) (5)
I:padding:checking that padding can be filtered out (6)
I:padding:checking that a TCP and padding server config enables padding (7)
I:padding:checking that a padding server config should enforce TCP (8)
I:padding:failed
I:padding:checking that zero-length padding option has no effect (9)
I:padding:exit status: 1
R:padding:FAIL