[tests] Get mailbox test running again

This commit fixes the mailbox tests in test_runner, which broke when
mailbox was simplified to just use call and respond. It also fixes a
bug the tests uncovered: if the mailbox is closed while a caller is in
the reply map (ie, when its call data has been passed on to a thread
calling respond, but has yet to be responded to itself), that caller is
never awoken.

assets/manifests/test.yaml

@@ -9,5 +9,3 @@ panic:
   - panic.serial
 services:
   - test_runner
-drivers:
-  - drv.uart