MDEV-17058: Test failure on wsrep.variables
MDEV-17060: Test failure on galera.galera_var_slave_threads
Fix incorrect calculation of increased applier (slave) threads.
Note that increase change takes effect "immediately" but we should
use proper wait condition to wait it. Reducing the number of
slave threads is not immediate as thread will only exit after a
replication event.
Two changes were made to the test:
1) Suppress warning "Refusing exit for the last slave thread."
This warning was already suppressed, but on the wrong node.
2) The test occasionally fails because it expects that the
number of applier threads changes immediately after
changing the value of ```variable wsrep_slave_threads```.
Which is not true. This patch turns snippets like this:
```
SET GLOBAL wsrep_slave_threads = x;
SELECT COUNT(*) = x FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user';
```
Into proper wait_conditions:
```
SET GLOBAL wsrep_slave_threads = x;
let $wait_condition = SELECT COUNT(*) = x FROM ...;
--source include/wait_condition.inc
```
Note: At least one test is unstable, failing with the following:
./mtr --mysqld=--innodb-purge-threads=9 --big-test --no-reorder \
galera.galera_parallel_autoinc_largetrx galera.galera_var_slave_threads
The result difference is dependent on innodb_purge_threads.
