Final loose-EG, dig in and solve..

[goodboy]

A follow-thru from #390, we still have one left per the comment
https://github.com/goodboy/tractor/pull/390/files#r2282918001 in our
tests_cancellation suite.

Further there's a related .xfail() test from
d2ac9ec
which still hasn't been resolved.

Leaving this as a longer term follow up (likely to be dealt with in
a panic when trio finally removes the strict_exception_groups
flag XD) since none of it seem more then benign.