fix `%magic` output

call lsmagic without underscore

closes #3467
exercise `%magic` in tests
remove yields from test_magic

at request of @fperez
cleanup nt.assert usage in test_magic

use `assert_in` and `assert_equal` in place of a few `assert_true/false`
Merge pull request #3468 from minrk/lsmagic

fix `%magic` output