This change fixes constant test failure on RHEL 7 system with
CFLAGS='-march=native':
>---------------------------------------------------------------<
 2222: STP - flush the fdb and mdb when topology changed  FAILED
 ...
 ./stp.at:609: ovs-appctl fdb/show br0
 --- -
 +++ ./tests/testsuite.dir/at-groups/2222/stdout
 @@ -1,2 +1,3 @@
   port  VLAN  MAC                Age
   +LOCAL     1  00:0c:29:a0:27:d1   33
>---------------------------------------------------------------<

Long warps takes threads a chance to perform some work on each
step unlike multiple appctl calls. Also, code looks cleaner and
works faster.

CC: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Fixes: 427e9751

 ("tests: Add and improve stp tests.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>