selftests/tc-testing: Fix SFB db test

Setting very small value of db like 10ms introduces rounding errors when
converting to/from jiffies on some kernel configs. For example, on 250hz
the actual value will be set to 12ms which causes the test to fail:

 # $ sudo ./tdc.py  -d eth2 -e 3410
 #  -- ns/SubPlugin.__init__
 # Test 3410: Create SFB with db setting
 #
 # All test results:
 #
 # 1..1
 # not ok 1 3410 - Create SFB with db setting
 #         Could not match regex pattern. Verify command output:
 # qdisc sfb 1: root refcnt 2 rehash 600s db 12ms limit 1000p max 25p target 20p increment 0.000503548 decrement 4.57771e-05 penalty_rate 10pps penalty_burst 20p

Set the value to 100ms instead which currently seem to work on 100hz,
250hz, 300hz and 1000hz kernel configs.

Fixes: 6ad92dc5

 ("selftests/tc-testing: add selftests for sfb qdisc")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>