Add basic tests for Link(), with 20 errors

50 new page link tests, including
- links beginning with colon (explicitly not interwiki)
- links not beginning with colon (implicitly no interwiki)
- partially qualified links depending on either
  config.mylang or config.family
- fully qualified links which should not depend
  on config settings.

Of the 50 tests, 20 demonstrate errors in Link parsing.

The erronenous parse results are a 'success' in these
tests, thereby capturing the current results, to catch
any changes in the erroneous results which do not correct
the problem.

To see the errors, use:
  $ PYWIKIBOT2_TEST_SHOW_FAILURE=1 \
      python pwb.py tests/link_tests.py -v

Also, two new interwiki link tests.

Bug: 55133
Change-Id: I9a66856cedcedd69e39dabbc100871125db4eaed
Merge "Add basic tests for Link(), with 20 errors"