Multi-source: [tests] Basic tests to review and add to the multi_source suite

Attached are two tests with result files.

multisource.test is a basic test which creates 2-master replication setup and checks that replication from both masters works. The result file is synthetic; the test fails due to MDEV-554 (relay log problem), so the result file will need to be updated after the problem is fixed. Also, the test will need to be modified (suppression text) if anything is done about MDEV-551.

status_vars.test checks that Slave_heartbeat_period and Slave_received_heartbeats variables are local, while Slave_open_temp_tables is global. A part of the test is currently disabled because heartbeat variables don't work this way. Both the test and result files will need to be updated after MDEV-555 is implemented. The test can also be used to add checks for more variables if they are touched.

Additionally, the following can be added to the suite:

• a one-line addition to the existing syntax.test in MDEV-549;
• a test for RESET SLAVE in MDEV-548 – the test might need to be updated or not, depending on the final implementation; and result file needs to be generated;
• a test for skip_counter in MDEV-547, it should be usable as is, maybe with minor changes depending on the implementation;

There will be a few more tests later.