enable linger mode to unit test macros ASSERT, RETURN

I initially had linger mode off in the endlevel tags of these
macros, to make sure they didn't block the end of noninteractive
scenarios.

But it turns out its better to have it on. It doesn't block the
tests when they run noninteractively, and it means that if a test
fails, you can boot it up with --showgui, and then run :inspect
from linger mode to see what happened. If you don't have linger
mode, then even with --showgui the failed test closes out too
quickly.