[fix] issue when upgrading from werkzeug v2.0.3 to v2.1.0

In v2.1.0 werkzeug [1] fixed an issue [2] to keep relative redirect locations by
default [3].  Since relative locations are returned, we need to fix out test
cases to avoid AssertionErrors like this one::

    ======================================================================
    FAIL: test_index_html_get (tests.unit.test_webapp.ViewsTestCase)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
    File "/home/runner/work/searxng/searxng/tests/unit/test_webapp.py", line 105, in test_index_html_get
      self.assertEqual(result.location, 'http://localhost/search?q=test')
    AssertionError: '/search?q=test' != 'http://localhost/search?q=test'
    - /search?q=test
    + http://localhost/search?q=test

[1] https://werkzeug.palletsprojects.com/
[2] https://github.com/pallets/werkzeug/issues/2352 fixed in
[3] https://github.com/pallets/werkzeug/pull/2354

Related-to: https://github.com/searxng/searxng/pull/1039#issuecomment-1085538288


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>