tests/1_interlinking: Report line number after every test.

I strongly suspect, my interlinking tests would've not catched
regressions as expected.
tests/1_interlinking: Add new test for multiple link targets per line.

Currently failing, of course!
ftplugin/wretch.vim: Add interlinking sanity (even more!).
tests/1_interlinking: Update known good state for fixed issues.