Address timing issues in 'inline' system test.

Address timing issues in 'inline' system test.

"rndc signing -serial <value>" could take longer than a second to
complete.  Loop waiting for update to succeed.

For tests where "rndc signing -serial <value>" is supposed to not
succeed, repeatedly test that we don't get the new serial, then
test that we have the old value.  This should prevent false negatives.