tests: Fix race in 'balance-tcp bonding' test.

tests: Fix race in 'balance-tcp bonding' test.

Running the test in a tight loop could cause this test to fail after
about 5 runs, with some of the ports reporting "may_enable: false" in
the "ovs-appctl bond/show" output. This commit fixes the race condition
by waiting for may_enable to be true for all bond ports.

I suspect that LACP negotiation finishes, but the main thread doesn't
have a chance to enable the ports before we send the test packets.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>