In ava check add id-length exceptions from 8.x

[sergiitk]

'id-length': ['error', { exceptions: ['t'] }], in ava.js overrides 'id-length': ['error', { exceptions: ['i', 'j', 'k'] }], from 8.x.js.

It'd make sense to allow 'i', 'j', 'k' in ava tests too.