Comment 6 for bug 1697700

x.regression then fails the testcase check:

x.regression [ pass ] 6506

MTR's internal check of the test case 'x.regression' failed.
This means that the test case does not preserve the state that existed
before the test case was executed. Most likely the test case did not
do a proper clean-up. It could also be caused by the previous test run
by this thread, if the server wasn't restarted.
This is the diff of the states of the servers before and after the
test case was executed:
mysqltest: Logging to '/mnt/workspace/mysql-5.7-param/BUILD_TYPE/release/Host/ubuntu-zesty-64bit/build/mysql-test/var/tmp/check-mysqld_1.log'.
mysqltest: Results saved in '/mnt/workspace/mysql-5.7-param/BUILD_TYPE/release/Host/ubuntu-zesty-64bit/build/mysql-test/var/tmp/check-mysqld_1.result'.
mysqltest: Connecting to server localhost:13001 (socket /tmp/NYJBA6iqfc/mysqld.1.sock) as 'root', connection 'default', attempt 0 ...
mysqltest: ... Connected.
mysqltest: Start processing test commands from './include/check-testcase.test' ...
mysqltest: ... Done processing test commands.
--- /mnt/workspace/mysql-5.7-param/BUILD_TYPE/release/Host/ubuntu-zesty-64bit/build/mysql-test/var/tmp/check-mysqld_1.result 2017-06-15 10:30:22.378105113 +0300
+++ /mnt/workspace/mysql-5.7-param/BUILD_TYPE/release/Host/ubuntu-zesty-64bit/build/mysql-test/var/tmp/check-mysqld_1.reject 2017-06-15 10:30:29.205820935 +0300
@@ -3114,12 +3114,12 @@
 mysql.help_relation 0
 mysql.host NULL
 mysql.procs_priv 0
-mysql.tables_priv 3286698170
+mysql.tables_priv 562417445
 mysql.time_zone 2420313365
 mysql.time_zone_leap_second 3799112081
 mysql.time_zone_name 2268956709
 mysql.time_zone_transition 3895294076
 mysql.time_zone_transition_type 168184411
-mysql.user 3283710450
+mysql.user 2300035176
 Warnings:
 Error 1146 Table 'mysql.host' doesn't exist

mysqltest: Result content mismatch

The fix is to backport the following from 8.0:

commit 8617d643bcaaab36155aef35b39afd60c3bf3f30
Author: Lukasz Kotula <email address hidden>
Date: Thu Jan 26 11:29:16 2017 +0100

    Fix for toggling x.regression

    Description
    ===========

    Creation of 'mysqlxsys' account in thread That calls plugin entry
    Point is not possible, because it could be executed at the start mysqld.
    When mysqld starts the SQL service is not accessable. The creation
    of account was moved to a separate thread which races with 'mysql-test'
    pre / post-check.

    Fix
    ===

    Plugin is going to be loaded by mysqltest script instead from MTRs
    opt file. In this case we aren't in race with MTR.

    RB: 15320
    Reviewed-by: Andrzej Religa <email address hidden>