Problem:
--------
Various Linux distributions are running the MySQL Test Suite during
build. As known, MySQL test suite has some flaky tests. If a build
is unlucky and gets a sporadic test failure, it is cancelled and
in the worst case, the distribution is blocked from upgrading MySQL
to a new one. Even if MTR is run with --retry > 1 and all failed
tests pass on retry, exit code from MTR is non zero and build stops.

Solution:
---------
An MTR option named --report-unstable-tests is added which does the
following:
- Reports any test which has passed on at least one retry attempt
  in a separate category called "Unstable tests" in the summary.
  They will be in the format :
  <test_name>(<number_of_failures>/<number_of_attempts>)
- In case all failures are due to unstable tests, it causes MTR
  to print a warning and terminate with a zero exit code.

Note:
This is a backport of the patch for Bug#24473420 from mysql-8.0

Reviewed-by: Pavan Naik <pavan.naik@oracle.com>