/usr/lib/mysql-testsuite/r/have_cp866.require 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 | Collation Charset Id Default Compiled Sortlen
cp866_general_ci cp866 36 Yes Yes 1
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