vtc: try to stabilize b84

from vtest:

**   top   === process p1 -stop -wait
**** dT    1.431
**** p1    Sent signal 15
**** p1    stdout read 0
**** p1    stderr read 0
**   p1    WAIT4 pid=26430 status=0x000f (user 0.004683 sys 0.009366)
...
**** top   shell_cmd|\ttest ! -f "/root/VT/_vtest_tmp/vtc.26358.31f097e1"/v2/_.secret
**** dT    2.455
**** top   shell_status = 0x0001
---- top   shell_exit not as expected: got 0x0001 wanted 0x0000

Not exactly sure why we see a status != 0 here, but maybe the kill is too early?

bin/varnishtest/tests/b00084.vtc

@@ -35,6 +35,8 @@ shell -match _.secret {
 	find "${tmpdir}"/v2/_.secret -perm 0640 -size 256c
 }
 
+shell "varnishadm -n ${tmpdir}/v2 ping"
+
 process p1 -stop -wait
 
 shell {