BUG#47983: rpl_extraColmaster_myisam failed in PB2 with "Found
warnings!!"

BUG 45214 fixed the case when get_master_version_and_clock
function, used by the slave, would not report errors. The slave
now detects them and if related to transient network failures, it
prints some warnings and retries to connect. On the other hand,
if not network related, it just gives up and fails.

As such, sometimes, in PB2, the slave comes across some transient
communication issues between master and slave, while calling
get_master_version_and_clock, causing warnings print outs to the
error log. Nevertheless, in such cases slave retries to connect,
in which it succeeds, and the test case continues as it normally
would. But then, at the end of a successful test run, MTR checks
the error log, finds the unexpected warnings and considers them
harmful. This causes MTR to report error and, consequently, PB2
to report a failing test.

We fix this by adding to the global warnings suppress list the
warnings related to transient network failures only, which are
reported while in function get_master_version_and_clock.