/usr/lib/mysql-testsuite/t/key_cache-master.opt is in percona-server-test-5.6 5.6.22-rel71.0-0ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | --key_buffer_size=2M --small.key_buffer_size=256K --small.key_buffer_size=128K
--force-restart
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