Don't use obsolete longlines-mode

longlines.el has been marked as obsolete in Emacs snapshot. This commit
replaces the reference to this mode by `abbrev-mode` for the
`when-i-load-the-following` unit-test. The behavior of the mode itself
is meaningless as this test only makes sure that the mode can be
activated.
Merge branch 'longlines-mode-obsolete'