MTR supports nested test suites, such as suite/engines/funcs.
It is run by providing the suite value as
Overlays do not support this. If I create storage/<engine>/mysql-test/engines/funcs and then run
it dies at lib/mtr_cases.pm line 383.
It seems that the change below fixes it, although I might fail to predict some deeper consequences.
Please replace the death message with something appropriate, but it's better to say something than die quietly.