mail/mu: update to 1.4.15

[dereks_lifeofadishwasher.com]

Summary

• Update mu and mu4e to 1.4.15
• Add test target
• UPDATING about mail/mu moving to ~/.cache/mu

Test Plan

testport mail/mu and mail/mu4e
run new test via 'make test'
use mu for a couple days on my system

[dereks_lifeofadishwasher.com]

I've been using 1.4.13 for a couple days now without issues.

[dereks_lifeofadishwasher.com]

Do you have any way to verify mu4e? I do not use it.

[dereks_lifeofadishwasher.com]

Change to 1.4.15