frang: improve tests for `request_rate` and `request_burst`

[RomanBelozerov]

This issue is necessary to reproduce 2286 issue.

The main problem in 2286 - sometimes, under heavy load, we don't drop all messages from one connection.

The test should check that warnings doesn't contain many messages exceeding the limit for a single connection.
Not allow:

[  518.131111] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 22 (lim=20) conn=ffff9aa79a6f9e18
[  518.131299] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 23 (lim=20) conn=ffff9aa79a6f9e18
[  518.131316] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 24 (lim=20) conn=ffff9aa79a6f9e18
[  518.131335] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 25 (lim=20) conn=ffff9aa79a6f9e18
[  518.131359] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 26 (lim=20) conn=ffff9aa79a6f9e18
[  518.131375] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 27 (lim=20) conn=ffff9aa79a6f9e18
[  518.131406] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.48: 28 (lim=20) conn=ffff9aa79a6f9e18

conn=ffff9aa79a6f9e18 - a connection pointer to a connection

Allow:

[  393.058252] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 6 (lim=5) conn=ffff8ff5ca15c848
[  393.080947] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 7 (lim=5) conn=ffff8ff5cc1f4848
[  393.092168] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 8 (lim=5) conn=ffff8ff5c9beb838
[  394.156705] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 6 (lim=5) conn=ffff8ff6f0c21818
[  394.335053] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 6 (lim=5) conn=ffff8ff6d4822020
[  394.348021] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 7 (lim=5) conn=ffff8ff5cc1f4848
[  395.315961] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 6 (lim=5) conn=ffff8ff5c9bfc040
[  395.315983] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 7 (lim=5) conn=ffff8ff5c9bfe060
[  395.319843] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 8 (lim=5) conn=ffff8ff6d52fb030
[  395.330757] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 9 (lim=5) conn=ffff8ff5c94ec848
[  395.331184] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 10 (lim=5) conn=ffff8ff6d4823838
[  395.363830] [tempesta fw] Warning: frang: requests burst exceeded for 192.168.123.39: 11 (lim=5) conn=ffff8ff6b6de8000

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