MDEV-504 fails on appveyor build with a warning from tablecache mutex

From https://ci.appveyor.com/project/vaintroub/server/build/build-5~branch-10.2

main.mdev-504 [ fail ] Found warnings/errors in server log file!
Test ended at 2018-02-01 06:26:11
line
2018-02-01 6:24:15 1596 [Warning] Detected table cache mutex contention at instance 4: 28% waits. Additional table cache instance cannot be activated: consider raising table_open_cache_instances. Number of active instances: 8.
^ Found warnings in C:/projects/server/win_build/mysql-test/var/log/mysqld.1.err
ok

• saving 'C:/projects/server/win_build/mysql-test/var/log/main.mdev-504/' to 'C:/projects/server/win_build/mysql-test/var/log/main.mdev-504/'