MDEV-7037: MariaDB 10.0 does not build on Debian / kfreebsd-i386/amd64 due to MTR failure: multi_source.gtid
MDEV-7106: Sporadic test failure in multi_source.gtid
MDEV-7153: Yet another sporadic failure of multi_source.gtid in buildbot

This patch fixes three races in the multi_source.gtid test case that could
cause sporadic failures:

1. Do not put SHOW ALL SLAVES STATUS in the output, the output is not stable.

2. Ensure that slave1 has replicated as far as expected, before stopping its
connection to master1 (otherwise the following wait will time out due to rows
not replicated from master1).

3. Ensure that slave2 has replicated far enough before connecting slave1 to it
(otherwise we get an error during connect that slave1 is ahead of slave2).